diff --git a/.JuliaFormatter.toml b/.JuliaFormatter.toml
index 73b8d7a6..99c86b02 100644
--- a/.JuliaFormatter.toml
+++ b/.JuliaFormatter.toml
@@ -7,7 +7,7 @@ annotate_untyped_fields_with_any = false
 conditional_to_if = true
 format_docstrings = true
 join_lines_based_on_source = true
-long_to_short_function_def = true
+long_to_short_function_def = false
 margin = 100
 separate_kwargs_with_semicolon = true
 short_to_long_function_def = true
diff --git a/src/Fit/Fit.jl b/src/Fit/Fit.jl
index 7dc752ae..aaa8e319 100644
--- a/src/Fit/Fit.jl
+++ b/src/Fit/Fit.jl
@@ -7,12 +7,12 @@ Functions for fitting to data.
 """
 module Fit
 using Compat: @compat
-@compat public fit_spectrum, ews_to_abundances, ews_to_stellar_parameters,
-               ews_to_stellar_parameters_direct
-
 using ..Korg, ForwardDiff
 
 include("fit_via_synthesis.jl")
 include("fit_via_EWs.jl")
+include("fit_via_multimethod.jl")
 
+@compat public fit_spectrum, ews_to_abundances, ews_to_stellar_parameters,
+               ews_to_stellar_parameters_direct, multimethod_abundances
 end # module
diff --git a/src/Fit/fit_via_multimethod.jl b/src/Fit/fit_via_multimethod.jl
new file mode 100644
index 00000000..923a6874
--- /dev/null
+++ b/src/Fit/fit_via_multimethod.jl
@@ -0,0 +1,141 @@
+using ..Korg
+using Statistics: mean, std
+using Interpolations: linear_interpolation
+using Trapz: trapz
+using ProgressMeter
+
+"""
+TODO
+TODO don't hardcode Fe / [m/H]
+TODO make sure solar abundances are flexible. I think they currently are.
+
+# Keyword arguments:
+
+  - `verbose` (default: `true`: whether to print progress information
+
+# TODO kwarg everything?
+"""
+function multimethod_abundances(linelist, element, guess, lines_to_fit, obs_flux, obs_wls, R,
+                                err=ones(length(obs_flux)); verbose=true,
+                                abundance_perturbations=-0.6:0.3:0.6, synth_kwargs...)
+    # TODO check no conflicting R specification
+    # check that lines_to_fit are within the range of obs_wls
+
+    # wavelengths, windows and LSF, TODO refactor
+    windows = [(l - 1, l + 1) for l in lines_to_fit] # TODO kwarg
+    merged_windows, _ = Korg.merge_bounds(windows, 0.0)
+    synthesis_wls, obs_wl_mask, LSF = Korg.Fit._setup_wavelengths_and_LSF(obs_wls,
+                                                                          nothing,
+                                                                          nothing, R,
+                                                                          merged_windows,
+                                                                          0)
+
+    if !issorted(lines_to_fit)
+        lines_to_fit = sort(lines_to_fit)
+    end
+    if lines_to_fit[1] < obs_wls[1] || lines_to_fit[end] > obs_wls[end]
+        throw(ArgumentError("lines_to_fit includes lines outside of obs_wls"))
+    end
+
+    obs_flux = obs_flux[obs_wl_mask]
+    err = err[obs_wl_mask]
+    obs_wls = obs_wls[obs_wl_mask]
+    line_indices = map(lines_to_fit) do λ
+        # this is dumb
+        searchsortedfirst(obs_wls, λ - 0.2):searchsortedlast(obs_wls, λ + 0.2)
+    end
+
+    linelist = Korg.filter_linelist(linelist, synthesis_wls, 10.0; warn_about_time=false)
+    abundances = abundance_perturbations .+ guess
+
+    # would be cool to multithread
+    model_spectra = @showprogress "synthesizing" ennabled=verbose map(abundances) do X_H
+        params = Dict([
+                          :wavelengths => synthesis_wls,
+                          :linelist => linelist,
+                          Symbol(element) => X_H,
+                          synth_kwargs...
+                      ])
+        flux = synth(; params...)[2]
+        LSF * flux
+    end
+    model_spectra = hcat(model_spectra...) # shape (n_wls, n_models)
+
+    # TODO better
+    pseudocontinuum_mask = abs.(model_spectra[:, end] - model_spectra[:, 1]) .< 0.01 # TODO kwarg
+    model_pseudocontinua = [calculate_pseudocontinuum(obs_wls, m, pseudocontinuum_mask)
+                            for m in eachcol(model_spectra)]
+    model_pseudocontinua = hcat(model_pseudocontinua...)
+    obs_pseudocontinuum = calculate_pseudocontinuum(obs_wls, obs_flux, pseudocontinuum_mask)
+
+    EWs = map(eachcol(model_spectra), eachcol(model_pseudocontinua)) do m, p
+        calculate_rough_EWs(obs_wls, m, p, line_indices)
+    end
+    EWs = hcat(EWs...)
+    obs_EWs = calculate_rough_EWs(obs_wls, obs_flux, obs_pseudocontinuum, line_indices)
+    EW_itps = interpolate_and_predict(abundances, obs_EWs, EWs)
+    EW_As = [itp(EW) for (itp, EW) in zip(EW_itps, obs_EWs)]
+
+    zscores = ((obs_flux .- model_spectra) ./ err) .^ 2
+    chi2 = Matrix{Float64}(undef, length(line_indices), length(abundances))
+    for (i, r) in enumerate(line_indices)
+        chi2[i, :] .= sum(zscores[r, :]; dims=1)[:]
+    end
+    chi2_itps, chi2_coeffs = quadratic_minimizers(abundances, chi2)
+    chi2_As = -chi2_coeffs[2, :] ./ 2chi2_coeffs[3, :]
+
+    depths = [calculate_line_core_depths(m, pc, line_indices)
+              for (m, pc) in zip(eachcol(model_spectra), eachcol(model_pseudocontinua))]
+    depths = hcat(depths...)
+    obs_depths = calculate_line_core_depths(obs_flux, obs_pseudocontinuum, line_indices)
+    depth_itps = interpolate_and_predict(abundances, obs_depths, depths)
+    depth_As = [itp.(d) for (itp, d) in zip(depth_itps, obs_depths)]
+
+    (; EW_As, depth_As, chi2_As, obs_wl_mask, model_spectra, pseudocontinuum_mask,
+     obs_pseudocontinuum, EWs, obs_EWs, EW_itps, line_indices, model_pseudocontinua, chi2,
+     chi2_coeffs, chi2_itps, depths, obs_depths, depth_itps)
+end
+
+"""
+Fit a quadratic model to chi2(abundance_perturbation), and report the minimizer
+"""
+function quadratic_minimizers(abundance_perturbations, chi2)
+    # fit a quadratic model
+    A = hcat((abundance_perturbations .^ i for i in 0:2)...)
+    coeffs = A \ chi2'
+    map(eachcol(coeffs)) do c
+        x -> sum(c .* x .^ (0:2))
+    end, coeffs
+end
+
+function interpolate_and_predict(abundance_perturbations, observed_quantities, model_quantities)
+    map(eachrow(model_quantities)) do quantities
+        A = hcat((quantities .^ i for i in 0:3)...)
+        coeffs = A \ abundance_perturbations
+        x -> sum(coeffs .* x .^ (0:3))
+    end
+end
+
+function calculate_pseudocontinuum(obs_wls, flux, pseudocontinuum_mask)
+    linear_interpolation(obs_wls[pseudocontinuum_mask], flux[pseudocontinuum_mask];
+                         extrapolation_bc=1.0)(obs_wls)
+end
+
+function calculate_rough_EWs(obs_wls, spectrum, pseudocontinuum, line_indices)
+    absorption = pseudocontinuum - spectrum
+    map(line_indices) do r
+        trapz(obs_wls[r], absorption[r]) * 1e3 # convert to mÅ
+    end
+end
+
+function calculate_line_core_depths(spectrum, pseudocontinuum, line_indices)
+    map(line_indices) do r
+        # get the middle 5 pixels in the range
+        # TODO everthing about this is terrible
+        firstind = r[1] + length(r) ÷ 2 - 2
+        lastind = firstind + 4
+        firstind = max(1, firstind)
+        lastind = min(length(spectrum), lastind)
+        mean(spectrum[firstind:lastind] - pseudocontinuum[firstind:lastind])
+    end
+end
diff --git a/src/synthesize.jl b/src/synthesize.jl
index c620c05a..133a1bfc 100644
--- a/src/synthesize.jl
+++ b/src/synthesize.jl
@@ -314,21 +314,43 @@ end
     filter_linelist(linelist, wls, line_buffer)
 
 Return a new linelist containing only lines within the provided wavelength ranges.
+
+# Arguments:
+
+  - `linelist`: The input linelist to filter.
+  - `wls`: The wavelengths to filter the linelist by. Type: [`Korg.Wavelengths`](@ref)
+  - `line_buffer`: The buffer around each wavelength range to include in the filtered linelist. (Default: 10 Å)
+
+# Keyword Arguments:
+
+  - `warn_empty`: Whether to warn if the filtered linelist is empty.
+  - `warn_about_time`: Whether to warn if the filtering process takes a long time.
 """
-function filter_linelist(linelist, wls, line_buffer; warn_empty=true)
+function filter_linelist(linelist, wls, line_buffer::AbstractFloat=10.0;
+                         warn_empty=true, warn_about_time=true)
+    if line_buffer > 1 # it's in Å
+        line_buffer *= 1e-8
+    end
     nlines_before = length(linelist)
 
     last_line_index = 0 # we need to keep track of this across iterations to avoid double-counting lines.
-    sub_ranges = map(eachwindow(wls)) do (λstart, λstop)
-        first_line_index = searchsortedfirst(linelist, (; wl=λstart - line_buffer); by=l -> l.wl)
-        # ensure we don't double-count lines.
-        first_line_index = max(first_line_index, last_line_index + 1)
+    t = @elapsed begin
+        sub_ranges = map(eachwindow(wls)) do (λstart, λstop)
+            first_line_index = searchsortedfirst(linelist, (; wl=λstart - line_buffer);
+                                                 by=l -> l.wl)
+            # ensure we don't double-count lines.
+            first_line_index = max(first_line_index, last_line_index + 1)
+
+            last_line_index = searchsortedlast(linelist, (; wl=λstop + line_buffer); by=l -> l.wl)
 
-        last_line_index = searchsortedlast(linelist, (; wl=λstop + line_buffer); by=l -> l.wl)
+            first_line_index:last_line_index
+        end
+        linelist = vcat((linelist[r] for r in sub_ranges)...)
+    end
 
-        first_line_index:last_line_index
+    if t > 0.2 && warn_about_time
+        @warn "Filtering the linelist to the requested wavelength range took $t seconds. Consider prefiltering the linelist with Korg.filter_linelist to avoid duplicating this work many times."
     end
-    linelist = vcat((linelist[r] for r in sub_ranges)...)
 
     if nlines_before != 0 && length(linelist) == 0 && warn_empty
         @warn "The provided linelist was not empty, but none of the lines were within the provided wavelength range."
diff --git a/src/utils.jl b/src/utils.jl
index b85e9ce1..b4129057 100644
--- a/src/utils.jl
+++ b/src/utils.jl
@@ -1,4 +1,5 @@
 using Statistics: quantile
+using Core: Argument
 using Interpolations: linear_interpolation, Flat
 using SparseArrays: spzeros
 
@@ -40,7 +41,12 @@ end
 
 # Convert R to a value based on its type
 # used in `_lsf_bounds_and_kernel`
-_resolve_R(R::Real, λ0) = R
+function _resolve_R(R::Real, λ0)
+    if !isfinite(R)
+        throw(ArgumentError("R (resolving power) must be a finite number (is $R)"))
+    end
+    R
+end
 _resolve_R(R::Function, λ0) = R(λ0 * 1e8)  # R is a function of λ in Å
 
 # Core LSF calculation shared by all variants
diff --git a/test/fit.jl b/test/fit.jl
index ece588a6..527474d7 100644
--- a/test/fit.jl
+++ b/test/fit.jl
@@ -398,4 +398,29 @@ using Random
             @test_throws m Korg.Fit.ews_to_stellar_parameters(linelist, erroneous_EWs)
         end
     end
+
+    @testset "MultiMethod" begin
+        # synthesize a spectrum and get the answer back.
+        # Use synthesize to make this a better integration test,
+        # because multi_method_abundaces is based on synth
+
+        linelist = Korg.get_VALD_solar_linelist()
+        A_X = format_A_X(-0.3, -0.2, Dict("N" => -0.1))
+        atm = interpolate_marcs(5777, 4.44, A_X)
+        sol = synthesize(atm, linelist, A_X, 5000, 5500)
+
+        fe_lines = Korg.air_to_vacuum.([5044.211 5054.642 5127.359 5127.679 5198.711 5225.525 5242.491 5247.05 5250.208 5295.312 5322.041 5373.709 5379.574 5386.334 5466.396 5466.987][:])
+
+        obs_wl = sol.wavelengths
+        obs_flux = Korg.apply_LSF(sol.flux ./ sol.cntm, sol.wavelengths, 100_000)
+
+        out = Korg.Fit.multimethod_abundances(linelist, "m_H", 0.0, fe_lines, obs_flux, obs_wl,
+                                              100_000;
+                                              abundance_perturbations, alpha_H=-0.2, N=-0.1,
+                                              Teff=5777, logg=4.44)
+
+        @test all(out.EW_As ≈ -0.3) atol=0.001
+        @test all(out.depth_As ≈ -0.3) atol=0.001
+        @test all(out.chi2_As ≈ -0.3) atol=0.001
+    end
 end