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
Make the soup base
Melt the butter in a pot until it bubbles.
Add the sliced onions and simmer for 30 minutes without browning.
Stir in the flour and let it thicken.
Gradually pour in the boiling milk, seasoning with salt and pepper.
Finish and serve
Press the mixture through a purée sieve and return to the heat.
Beat the egg yolks and cream together.
Remove the soup from the stove and stir in the egg and cream mixture quickly.
Pour into a tureen 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.
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
- Soften and sweeten as they simmer, forming the body of the soup.
- Flour
- Thickens the milk and butter mixture into a smooth, body-rich base.
- Milk
- Forms the liquid base that the onions and flour thicken into soup.
- Egg yolks
- Enrich and thicken the finished soup when stirred in off the heat.
- Cream
- Adds richness and a smooth, silky mouthfeel to the soup.
The step that matters
Stir in the egg and cream mixture quickly, off the heat. Egg yolks thicken by gentle heat, but if they get too hot they can scramble into specks. Removing the pot from the stove and stirring fast spreads the yolks evenly before they set.
Swap the onion
How well each one fits the dish's main ingredients (butter, flour, milk, pepper, egg, cream):
- Potatofits better
- Asparagusfits better
- Leekfits better
- Onionthe original
Fit is the average pairing score against butter, flour, milk, pepper, egg, 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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