Why it works: Onion and mustard match in chemistry and in kitchens. They rank in the top 6% of all ingredient pairs, and cooks put them together in 9,259 recipes. Both carry 2-isopropyl-3-methoxypyrazine, which smells earthy, pea. The science ↓
Sauce Robert
Ingredients
Directions
Make the sauce
Melt the butter in a saucepan over low heat.
Add the chopped onions and cook until browned.
Sprinkle the flour over the onions and stir well.
Gradually add the stock while stirring continuously.
Leave the saucepan uncovered and reduce the sauce for 20 minutes.
Remove the saucepan from the heat.
Finish and serve
In a bowl, mix the mustard with the vinegar, adding it drop by drop.
Stir the mustard mixture into the sauce and serve in a sauceboat.
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.
Explore the science
The flavor team
The pairs that carry this dish. Each one is judged two ways: how much their aromas overlap, and how often cooks already put them together.
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Onion + mustardtop 6% of all pairsSmell alikestrongCooked together9,259 recipes
Shared aroma: 2-isopropyl-3-methoxypyrazine (earthy, pea).
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Mustard + vinegartop 7% of all pairsSmell alikemoderateCooked together14,067 recipes
Shared aroma: isobutyraldehyde (malty, green).
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Butter + flourtop 12% of all pairsSmell alikelowCooked together124,287 recipes
Shared aroma: isobutyraldehyde (malty, green).
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Flour + mustardtop 13% of all pairsSmell alikemoderateCooked together5,066 recipes
Shared aroma: isobutyraldehyde (malty, green).
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Onion + vinegartop 14% of all pairsSmell alikemoderateCooked together18,226 recipes
Shared aroma: acetic acid (sour, vinegar).
Butter and flour barely smell alike, yet cooks put them together in 124,287 recipes: they 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
- Onions
- Browned onions build the savoury base and deep flavour of the sauce.
- Butter
- Fat that cooks the onions and carries flavour through the sauce.
- Flour
- Starch that thickens the stock into a smooth, cohesive sauce.
- Stock
- Liquid that forms the body of the sauce as it reduces.
- Mustard
- Adds sharp, pungent flavour that cuts through the richness of the butter.
- Vinegar
- Adds acidity that brightens the sauce and balances its richness.
The step that matters
Adding the flour and stock. Flour must be stirred evenly into the fat and onions before liquid is added, and stock added gradually while stirring, so starch disperses without lumps and the sauce thickens smoothly.
Swap the onion
How well each one fits the dish's main ingredients (butter, flour, mustard, vinegar):
- Shallotfits better
- Horseradishfits better
- Radishfits better
- Onionthe original
Fit is the average pairing score against butter, flour, mustard, vinegar. Candidates are from the same food group and used in at least 100 recipes.
“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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