Why it works: Milk and butter 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
Cook the onions
Melt the butter in a frying pan.
Slice the onions thinly and add them to the butter.
Stir until they begin to cook, then cover and let simmer for 30 minutes.
Add the flour to the onions and stir constantly for 3 minutes over the heat.
Make the soup
Bring the milk to a boil.
Pour the onion mixture into the boiling milk and cook for 15 minutes.
Strain the soup and return it to the heat, seasoning with salt and pepper.
Beat the egg yolks, add the cream, and stir into the soup.
Cook for 3 minutes, stirring constantly.
If you don't have cream, use milk and add 1 tablespoon of butter.
Serve
Serve over fried croutons in a soup tureen.
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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Milk + buttertop 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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Milk + flourtop 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 release sweetness as they simmer, forming the body of the soup.
- Butter
- Carries the onion flavour and adds richness when cooking the onions.
- Flour
- Thickens the onion mixture so the soup becomes smooth and body-rich.
- Milk
- Forms the liquid base of the soup and adds creaminess.
- Egg yolks
- Enrich and thicken the soup as they gently set with heat.
- Cream
- Adds richness and a silkier texture to the finished soup.
The step that matters
Adding the egg yolks. Egg yolks thicken by gently setting with heat. If the soup is too hot when they go in, or it is boiling, they can curdle and turn grainy. Stir steadily and keep the heat low for a smooth result.
Swap the onion
How well each one fits the dish's main ingredients (milk, egg, butter, flour, cream, pepper):
- Potatofits better
- Asparagusfits better
- Leekfits better
- Onionthe original
Fit is the average pairing score against milk, egg, butter, flour, cream, pepper. 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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