Why it works: Onion and mustard match in chemistry and in kitchens. They rank in the top 6% of all ingredient pairs, and 9,259 recipes mention both. Both carry 2-isopropyl-3-methoxypyrazine, which smells earthy, pea. The science ↓
Brown Sauce
Ingredients
Directions
Make the brown sauce
Heat the butter or dripping in a small pan until hot.
Slice the onion and fry it in the fat until brown.
Stir in the flour, add the gravy or water, and let it boil.
Add the mustard and a few drops of vinegar, ketchup, or Harvey sauce; boil for one minute.
Strain the sauce to remove the onion.
Make the egg sauce
Boil the egg for ten minutes, then remove the shell and chop it.
In a small saucepan, mix the butter and flour over heat.
Pour in the milk and stir until it boils.
Add the chopped egg, mix well, and serve.
Transcribed from a scanned cookbook and modernised for today's kitchens. Where the book gave no heat or time, or said "a moderate oven", the temperatures and times here may be modern equivalents rather than the book's own figures.
Photo for illustrative purposes only · Contact us to use it
Explore the science
The flavor team
The pairs that carry this dish. Each one is judged two ways: how much aroma chemistry they share, and how many recipes mention both.
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Onion + mustardtop 6% of all pairsAroma overlapstrongMentioned together9,259 recipes
Shared aroma: 2-isopropyl-3-methoxypyrazine (earthy, pea).
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Butter + milktop 7% of all pairsAroma overlapmoderateMentioned together81,283 recipes
Shared aroma: (z)-4-heptenal (biscuit, cream).
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Mustard + vinegartop 7% of all pairsAroma overlapmoderateMentioned together14,067 recipes
Shared aroma: isobutyraldehyde (malty, green).
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Butter + flourtop 12% of all pairsAroma overlaplowMentioned together124,287 recipes
Shared aroma: isobutyraldehyde (malty, green).
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Flour + mustardtop 13% of all pairsAroma overlapmoderateMentioned together5,066 recipes
Shared aroma: isobutyraldehyde (malty, green).
Butter and flour share little aroma chemistry, yet 124,287 recipes mention both: they likely work on texture and structure more than aroma.
Ranks compare each pair with 20,000 random ingredient pairs on CompKitchen. How we score pairs
What each ingredient does
- Onion
- Frying until brown gives the sauce a deeper colour and savoury flavour, and it is strained out afterwards.
- Flour
- Thickens the sauce as it cooks with the fat and liquid.
- Gravy or water
- Forms the liquid base of the sauce.
- Egg
- Chopped and stirred into the milk sauce, adding body and texture.
- Milk
- Makes the white sauce base that the chopped egg is added to.
The step that matters
Stirring the flour into the milk sauce. Flour must be cooked with fat and liquid while stirring constantly, or it forms lumps and the sauce thickens unevenly. Bringing the milk to a boil while stirring lets the starch swell and thicken fully.
Flavor stand-ins for the onion
How well each one fits the dish's main ingredients (butter, flour, mustard, vinegar, egg, milk):
- Shallotbetter flavor fit
- Chivebetter flavor fit
- Horseradishbetter flavor fit
- Onionthe original
Fit is the average pairing score against butter, flour, mustard, vinegar, egg, milk. Candidates are from the same food group and used in at least 100 recipes. This compares flavor only: it does not check the job the ingredient does in the dish (bulk, moisture, acidity, structure).
“What each ingredient does” and “The step that matters” were written by Claude from this recipe’s own method and reviewed in samples by CompKitchen.
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