Name: Onion Soup
Ingredients: 1 oz Butter; 1 oz Flour; ½ cup Milk; 2 quarts Stock; Salt to taste; Pepper to taste; Few drops of lemon juice; Fried bread for serving

Why it works: Pepper and lemon match in chemistry and in kitchens. They rank in the top 3% of all ingredient pairs, and cooks put them together in 27,047 recipes. Both carry trans-2-nonenal, which smells cardboard, fatty. The science ↓

Onion Soup

Ingredients

Directions

Peel and slice the onions.
Fry the onions in the butter until they are a good brown color.
Sprinkle the flour over the onions and brown it as well.
Pour the boiling stock into the saucepan and boil steadily until the onions are very soft.
Rub the mixture through a sieve.
If there is any fat, remove it carefully.
Pour the soup back into the saucepan, add the milk, pepper, and salt, and bring to a boil.
Just before serving, add a few drops of lemon juice.
Serve with fried bread.

Recipe from a historical cookbook (Internet Archive).

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.

Pepper + lemontop 3% of all pairs

Smell alikestrong

Cooked together27,047 recipes

Shared aroma: trans-2-nonenal (cardboard, fatty).
Butter + breadtop 7% of all pairs

Smell alikemoderate

Cooked together27,232 recipes

Shared aroma: delta-decalactone (sweet, creamy).
Butter + milktop 7% of all pairs

Smell alikemoderate

Cooked together81,283 recipes

Shared aroma: (z)-4-heptenal (biscuit, cream).
Milk + breadtop 11% of all pairs

Smell alikemoderate

Cooked together11,466 recipes

Shared aroma: 2-undecenal (sweet).
Butter + flourtop 12% of all pairs

Smell alikelow

Cooked together124,287 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

Swap the lemon

How well each one fits the dish's main ingredients (butter, flour, milk, pepper, bread):

Figfits about as well
Lemonthe original
Vanillafits about as well
Raisinfits about as well

Fit is the average pairing score against butter, flour, milk, pepper, bread. Candidates are from the same food group and used in at least 100 recipes.