Why it works: Onion and potato are a kitchen classic. They rank in the top 3% of all ingredient pairs, and cooks put them together in 9,023 recipes. Both carry 2-isopropyl-3-methoxypyrazine, which smells earthy, pea. The science ↓
Onion Soup
Ingredients
Directions
Make the soup base
In a saucepan, melt the butter over medium heat.
Add the sliced onions and cook until they are nicely browned.
Stir in the flour and cook until slightly browned, being careful not to burn it.
Pour in the boiling water and add salt and pepper; mix well and let it boil for 1 minute.
Transfer the mixture to a soup kettle and keep warm on the back of the stove.
Finish and serve
Stir in the boiling milk and the mashed potatoes, adding a little soup to the potatoes first to thin them out.
Combine everything well, taste, and adjust seasoning if necessary.
Let it simmer for a few moments.
Place toasted bread in the bottom of a tureen, pour in the soup, and serve hot.
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 + potatotop 3% of all pairsSmell alikemoderateCooked together9,023 recipes
Shared aroma: 2-isopropyl-3-methoxypyrazine (earthy, pea).
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Butter + breadtop 7% of all pairsSmell alikemoderateCooked together27,232 recipes
Shared aroma: delta-decalactone (sweet, creamy).
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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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Pepper + potatotop 9% of all pairsSmell alikemoderateCooked together8,305 recipes
Shared aroma: 2-isopropyl-3-methoxypyrazine (earthy, pea).
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Milk + potatotop 9% of all pairsSmell alikemoderateCooked together3,611 recipes
Shared aroma: (e,z)-2,6-nonadienal (cucumber, melon).
Ranks compare each pair with 20,000 random ingredient pairs on CompKitchen. How we score pairs
What each ingredient does
- Onions
- Browning in butter builds the sweet, deep flavour that anchors the soup.
- Flour
- Cooked with the butter, it forms a roux that thickens the soup.
- Milk
- Adds richness and body, softening the soup's flavour and texture.
- Potatoes
- Mashed into the soup, they give extra body and a creamier texture.
- Butter
- Carries the onions and forms the base of the roux, adding richness.
The step that matters
Stir in the flour and cook until slightly browned. Flour must be cooked in the fat long enough to lose its raw taste, but not so long it burns. Burnt flour turns bitter and dark, and undercooked flour leaves a pasty flavour and weak thickening.
Swap the onion
How well each one fits the dish's main ingredients (butter, flour, pepper, milk, potato, bread):
- Capsicumfits better
- Chivefits better
- Asparagusfits better
- Onionthe original
Fit is the average pairing score against butter, flour, pepper, milk, potato, bread. 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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