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Onion Soup

A creamy soup made from onions, butter, milk, flour, and egg yolks.

From an old cookbook · modernised transcription
From a historical cookbook (Internet Archive) · 6 ingredients
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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 ↓

Onion Soup

Ingredients

Directions

Make the soup

  1. Slice the onions and fry them slowly in butter for 30 minutes.

  2. Add the flour and cook for 3 minutes, stirring constantly.

  3. Add the hot milk and cook for 15 minutes.

  4. Strain the mixture and return it to the heat.

Finish and serve

  1. Beat the egg yolks with the cold milk and add to the soup.

  2. Bring to a boil and serve.

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 + milktop 7% of all pairs
    Smell alikemoderate
    Cooked together81,283 recipes

    Shared aroma: (z)-4-heptenal (biscuit, cream).

  2. Butter + flourtop 12% of all pairs
    Smell alikelow
    Cooked together124,287 recipes

    Shared aroma: isobutyraldehyde (malty, green).

  3. Milk + flourtop 14% of all pairs
    Smell alikelow
    Cooked together78,219 recipes

    Shared aroma: isobutyraldehyde (malty, green).

  4. Onion + buttertop 15% of all pairs
    Smell alikemoderate
    Cooked together42,912 recipes

    Shared aroma: decanal (soap, orange peel).

  5. Flour + eggtop 15% of all pairs
    Smell alikelow
    Cooked together106,462 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
Slowly fried onions provide the main sweetness and savoury flavour of the soup.
Butter
Fat for frying the onions, carrying flavour and helping them brown gently.
Flour
Cooked with the butter, it thickens the milk into a smooth, body-rich soup.
Hot milk
Forms the liquid base of the soup, adding creaminess and mild richness.
Egg yolks
Beaten with cold milk and stirred in, they enrich and thicken the soup.

The step that matters

Adding the egg yolks. Egg yolks thicken when heated, but if the soup is too hot they can scramble and curdle. Beating them with cold milk first tempers them so they blend in smoothly.

Swap the onion

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

  1. Chivefits better
  2. Potatofits better
  3. Leekfits better
  4. Onionthe original

Fit is the average pairing score against butter, milk, flour, egg. 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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