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
The book gives no oven temperature. The 400°F (200°C) in the steps was added when the recipe was transcribed, as a starting point.
Make the soup base
Brown the onions in a saucepan with the butter.
Stir in the flour and moisten with the white broth.
Season with salt and pepper, and cook for 10 minutes.
Assemble and bake
Place a small square of toasted bread in a bowl.
Cover the bread with fine slices of Swiss cheese.
Pour the hot broth over the cheese and bread.
Add a few more slices of cheese on top.
Put in the oven at 400°F (200°C) for 5 minutes before serving.
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.
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Butter + breadtop 7% of all pairsSmell alikemoderateCooked together27,232 recipes
Shared aroma: delta-decalactone (sweet, creamy).
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Butter + swiss cheesetop 8% of all pairsSmell alikemoderateCooked together413 recipes
Shared aroma: delta-decalactone (sweet, creamy).
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Onion + peppertop 11% of all pairsSmell alikemoderateCooked together68,474 recipes
Shared aroma: 2-isopropyl-3-methoxypyrazine (earthy, pea).
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Butter + flourtop 12% of all pairsSmell alikelowCooked together124,287 recipes
Shared aroma: isobutyraldehyde (malty, green).
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Onion + buttertop 15% of all pairsSmell alikemoderateCooked together42,912 recipes
Shared aroma: decanal (soap, orange peel).
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
- Browning them builds the sweet, deep flavour that defines the soup.
- Butter
- Fat that carries the browning of the onions and adds richness.
- Flour
- Thickens the broth slightly so it clings to the bread and cheese.
- White broth
- Liquid base that gives the soup its body and savoury flavour.
- Swiss cheese
- Melts over the bread to add richness and a stretchy, gratinéed topping.
The step that matters
Browning the onions. Onions need time over moderate heat to caramelise fully; cooking them too fast scorches the sugars and leaves a bitter taste that carries into the whole soup.
Swap the onion
How well each one fits the dish's main ingredients (butter, flour, pepper, bread, swiss cheese):
- Potatofits better
- Capsicumfits better
- Asparagusfits better
- Onionthe original
Fit is the average pairing score against butter, flour, pepper, bread, swiss cheese. 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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