Name: Onion Soup
Ingredients: light stock, milk, or water; onions; bread-crumbs or barley-flour; optional: leeks, celery, parsley, sage

Why it works: Celery and parsley match in chemistry and in kitchens. They rank in the top 1% of all ingredient pairs, and cooks put them together in 6,810 recipes. Both carry 4'-methylacetophenone, which smells bitter almond. The science ↓

Onion Soup

Ingredients

light stock, milk, or water
onions
bread-crumbs or barley-flour
optional: leeks, celery, parsley, sage

Directions

Boil the onions until soft, then chop or sieve them.
Add the chopped onions to your choice of liquid (light stock, milk, or water).
Thicken the soup with bread-crumbs or barley-flour.
For variation, use leeks or a mix of onions and celery.
Adjust the consistency and proportions to taste.

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.

Celery + parsleytop 1% of all pairs

Smell alikestrong

Cooked together6,810 recipes

Shared aroma: 4'-methylacetophenone (bitter almond).
Parsley + sagetop 1% of all pairs

Smell alikestrong

Cooked together1,445 recipes

Shared aroma: beta-phellandrene (minty, terpene).
Leek + celerytop 4% of all pairs

Smell alikestrong

Cooked together1,093 recipes

Shared aroma: coumarin (sweet, hay).
Onion + leektop 4% of all pairs

Smell alikestrong

Cooked together1,326 recipes

Shared aroma: ethyl butyrate (apple, fruity).
Leek + parsleytop 4% of all pairs

Smell alikestrong

Cooked together759 recipes

Shared aroma: trans-2-hexenal (green, leafy).

Ranks compare each pair with 20,000 random ingredient pairs on CompKitchen. How we score pairs

Swap the leek

The recipe marks leek as optional. How well each one fits the dish's main ingredients (milk, onion, bread, celery, parsley, sage):

Carrotfits better
Potatofits about as well
Leekthe original
Turnipfits about as well

Fit is the average pairing score against milk, onion, bread, celery, parsley, sage. Candidates are from the same food group and used in at least 100 recipes.