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 fat in a pot over medium heat.
Add the finely chopped onions and cook slowly until soft.
Increase the heat and cook until the onions are browned but not burned.
Finish and serve
Add the boiling water or water and milk.
If desired, thicken the soup with flour mixed in a little cold water.
Bring to a boil and serve hot.
Optionally, pour the soup over browned bread and top with grated mild cheese.
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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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).
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Onion + buttertop 15% of all pairsSmell alikemoderateCooked together42,912 recipes
Shared aroma: decanal (soap, orange peel).
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
- Provide the main flavour and sweetness as they soften and brown in the fat.
- Fat
- Carries the onions' flavour and helps them brown evenly in the pot.
- Flour
- Thickens the soup slightly when mixed with cold water before adding.
- Water
- Forms the liquid base of the soup, extracting flavour from the browned onions.
The step that matters
Browning the onions. Cooking onions slowly until soft and then browning them over higher heat develops sweet, deep flavours through caramelisation. If they burn, the soup turns bitter, so the heat must be watched closely.
Swap the onion
How well each one fits the dish's main ingredients (pepper, butter, flour, milk):
- Potatofits better
- Capsicumfits better
- Chivefits better
- Onionthe original
Fit is the average pairing score against pepper, butter, flour, milk. 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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