Why it works: Butter and milk are a kitchen classic. They rank in the top 7% of all ingredient pairs, and cooks put them together in 81,283 recipes. Both carry (z)-4-heptenal, which smells biscuit, cream. The science ↓
Onion Soup
Ingredients
Directions
Prepare
Remove the skin from the onions and slice them thinly.
Make the soup
In a pot, melt the butter and add the onions; cook for 2 minutes.
Cover tightly and let simmer on low heat until the onions are soft but not browned.
Sprinkle the flour over the onions, stir until well mixed, and cook for 3 minutes, stirring constantly.
Add the milk and cook in a double boiler for 30 minutes.
Strain the soup through a sieve and add salt and pepper to taste.
Finish and serve
Beat the egg yolks slightly and gradually mix in the cream, then add to the soup just before serving.
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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Butter + milktop 7% of all pairsSmell alikemoderateCooked together81,283 recipes
Shared aroma: (z)-4-heptenal (biscuit, cream).
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Butter + creamtop 8% of all pairsSmell alikemoderateCooked together41,516 recipes
Shared aroma: delta-dodecalactone (sweet, creamy).
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Onion + peppertop 11% of all pairsSmell alikemoderateCooked together68,474 recipes
Shared aroma: 2-isopropyl-3-methoxypyrazine (earthy, pea).
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Butter + flourtop 12% of all pairsSmell alikelowCooked together124,287 recipes
Shared aroma: isobutyraldehyde (malty, green).
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Flour + milktop 14% of all pairsSmell alikelowCooked together78,219 recipes
Shared aroma: isobutyraldehyde (malty, green).
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
- Form the body of the soup and, when cooked slowly, add sweetness and savory depth.
- Butter
- Carries the onions' flavor and adds richness to the soup.
- Flour
- Thickens the soup as it cooks with the butter and onions.
- Milk
- Forms the liquid base of the soup and gives it a creamy body.
- Egg yolks
- Enrich and thicken the soup when stirred in at the end.
- Cream
- Adds richness and a smooth, velvety texture.
The step that matters
Adding the egg yolk and cream mixture. Egg yolks thicken when heated, so if the soup is too hot or boils after they go in, they can curdle and turn grainy. Beat them with the cream, add it gradually, and add it just before serving.
Swap the onion
How well each one fits the dish's main ingredients (butter, pepper, flour, egg, milk, cream):
- Potatofits better
- Asparagusfits better
- Leekfits better
- Onionthe original
Fit is the average pairing score against butter, pepper, flour, egg, milk, cream. 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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