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 ↓
Cream of Onion Soup
Ingredients
Directions
Make the soup base
Melt butter, add onions, and stir to mix.
Cover and cook slowly for 10–12 minutes until soft but not brown.
Place flour in another pan, add milk and seasonings, and bring to a boil, stirring the entire time.
Add the onion mixture. Reduce heat to low and simmer covered for 8–12 minutes.
Do not boil, as this will cause the soup to curdle because of acid in onions.
Finish with the liaison
Mix egg yolks with cream until smooth.
Stir 2 tablespoons hot soup into the yolks, then add a bit more hot soup to bring the yolks up to temperature.
Stir this back into the soup.
Reheat carefully over low heat until slightly thickened.
Check seasonings and adjust if needed.
Serve
Serve with a few croutons floating in bowls.
Nutrition, whole recipe (rough)
What these numbers assume: 8 portions
| Ingredient | Portion assumed | kcal |
|---|---|---|
| Flour | 1 cup (125g) all-purpose | 455 |
| Milk | 1 cup (244ml) whole | 149 |
| Butter | 1 tbsp (14g) | 102 |
| Egg | 1 large egg (50g) | 78 |
| Cream | 1 tbsp (15ml) heavy | 51 |
| Onion | 1 medium (110g) | 44 |
| Black Pepper | 1 tsp ground (2.3g) | 6 |
| Salt | 1 tsp (6g) | 0 |
One typical portion per ingredient; the recipe's quantities are not counted. A rough guide only, not for medical or dietary planning.
Steps grouped into stages for easier reading; the method is the source's own.
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.
-
Butter + milktop 7% of all pairsSmell alikemoderateCooked together81,283 recipes
Shared aroma: (z)-4-heptenal (biscuit, cream).
-
Butter + creamtop 8% of all pairsSmell alikemoderateCooked together41,516 recipes
Shared aroma: delta-dodecalactone (sweet, creamy).
-
Onion + peppertop 11% of all pairsSmell alikemoderateCooked together68,474 recipes
Shared aroma: 2-isopropyl-3-methoxypyrazine (earthy, pea).
-
Butter + flourtop 12% of all pairsSmell alikelowCooked together124,287 recipes
Shared aroma: isobutyraldehyde (malty, green).
-
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
- Cooked slowly, they turn soft and sweet, forming the soup's main flavor.
- Butter
- Adds richness and carries flavor as the onions cook gently.
- Flour
- Thickens the milk into a smooth, body-rich base.
- Milk
- Forms the liquid base that gives the soup its creamy body.
- Egg yolks
- Thicken the soup slightly and add richness when heated gently.
- Heavy cream
- Adds fat for a smooth, rich mouthfeel.
The step that matters
Keep the soup below a boil once the yolks go in. Egg yolks thicken by gentle heat and curdle if overheated, and the acid in onions makes curdling more likely. Tempering the yolks first and reheating over low heat keeps the soup smooth.
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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