Name: Onion Soup
Ingredients: 8 medium-sized onions, peeled; ¼ pound fresh butter; flour (as needed); 3 quarts veal broth; coarse sea salt (to taste); liaison (optional, for thickening)

Why it works: Butter and flour are a kitchen classic. They rank in the top 12% of all ingredient pairs, and cooks put them together in 124,287 recipes. Both carry isobutyraldehyde, which smells malty, green. The science ↓

Onion Soup

Ingredients

Directions

Prepare

Slice the onions very thinly.

Make the soup

In a large pot, melt the butter and add the onions and flour; cook until tender.
Pour in the veal broth and bring to a boil for 20 minutes.
Skim the surface, season with salt, and add the liaison.
Whisk well, let it simmer, and serve hot.

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.

Butter + flourtop 12% of all pairs

Smell alikelow

Cooked together124,287 recipes

Shared aroma: isobutyraldehyde (malty, green).
Onion + buttertop 15% of all pairs

Smell alikemoderate

Cooked together42,912 recipes

Shared aroma: decanal (soap, orange peel).
Onion + vealtop 17% of all pairs

Smell alikelow

Cooked together6,841 recipes

Shared aroma: 2,4-decadienal (seaweed).
Onion + flourtop 17% of all pairs

Smell alikemoderate

Cooked together32,460 recipes

Shared aroma: isobutyraldehyde (malty, green).
Butter + vealtop 23% of all pairs

Smell alikemoderate

Cooked together9,381 recipes

Shared aroma: 3-hexenal (apple, fruity).

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 body and sweetness as they cook down until tender.
Butter: Adds fat and richness, and helps the onions soften and brown gently.
Flour: Thickens the broth slightly and helps give the soup body.
Veal broth: Forms the liquid base and carries the savory flavor of the soup.