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Onion Soup

A soup made with butter, olive oil, sliced onion, water, and seasoned with coarse sea salt, served over thin slices of stale bread.

From an old cookbook · modernised transcription
From a historical cookbook (Internet Archive) · 6 ingredients
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Why it works: Butter and bread are a kitchen classic. They rank in the top 7% of all ingredient pairs, and cooks put them together in 27,232 recipes. Both carry delta-decalactone, which smells sweet, creamy. The science ↓

Onion Soup

Ingredients

Directions

Make the soup

  1. In a saucepan, combine butter and olive oil.

  2. Add the sliced onion and fry until golden brown.

  3. Pour in water and cook for 4 hours.

Season and serve

  1. Season with coarse sea salt to taste.

  2. Strain the soup over thin slices of stale 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.

  1. Butter + breadtop 7% of all pairs
    Smell alikemoderate
    Cooked together27,232 recipes

    Shared aroma: delta-decalactone (sweet, creamy).

  2. Olive + oniontop 8% of all pairs
    Smell alikemoderate
    Cooked together29,779 recipes

    Shared aroma: 2,4-decadienal (seaweed).

  3. Olive + breadtop 9% of all pairs
    Smell alikemoderate
    Cooked together1,091 recipes

    Shared aroma: 2-undecenal (sweet).

  4. Butter + oniontop 15% of all pairs
    Smell alikemoderate
    Cooked together42,912 recipes

    Shared aroma: decanal (soap, orange peel).

  5. Butter + olivetop 15% of all pairs
    Smell alikemoderate
    Cooked together11,887 recipes

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

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

What each ingredient does

Butter
Adds richness and flavour, and helps the onion brown in the pan.
Olive oil
Carries heat evenly during frying and keeps the onion from scorching too quickly.
Onion
Browns into sweet, deep flavour that gives the soup its character.
Stale bread
Soaks up the soup and gives it body as it is served.

The step that matters

Frying the onion until golden brown. Slow frying browns the onion's natural sugars and proteins, adding sweetness and depth. Stopping while it is still pale leaves the soup flat, and letting it burn adds bitterness.

Swap the onion

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

  1. Tomatofits better
  2. Capsicumfits better
  3. Potatofits about as well
  4. Onionthe original

Fit is the average pairing score against butter, olive, bread. 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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