Why it works: Butter and parmesan cheese are a kitchen classic. They rank in the top 9% of all ingredient pairs, and 4,163 recipes mention both. Both carry guaiacol, which smells smoky, sweet. The science ↓
Onion Soup Au Gratin
Ingredients
Directions
Make the soup
Fry the minced onions in butter until golden brown.
Add flour and blend well.
Stir in bouillon and cook for 10 minutes until smooth.
Season with salt and pepper.
Gratin and serve
Place slices of toast on top of the soup.
Sprinkle generously with Parmesan cheese and brown in the oven.
Serve very hot.
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 aroma chemistry they share, and how many recipes mention both.
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Butter + parmesan cheesetop 9% of all pairsAroma overlapmoderateMentioned together4,163 recipes
Shared aroma: guaiacol (smoky, sweet).
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Onion + peppertop 11% of all pairsAroma overlapmoderateMentioned together68,474 recipes
Shared aroma: 2-isopropyl-3-methoxypyrazine (earthy, pea).
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Butter + flourtop 12% of all pairsAroma overlaplowMentioned together124,287 recipes
Shared aroma: isobutyraldehyde (malty, green).
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Pepper + parmesan cheesetop 14% of all pairsAroma overlapmoderateMentioned together5,322 recipes
Shared aroma: ethyl 2-methylbutyrate (fruity, apple).
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Onion + buttertop 15% of all pairsAroma overlapmoderateMentioned together42,912 recipes
Shared aroma: decanal (soap, orange peel).
Butter and flour share little aroma chemistry, yet 124,287 recipes mention both: they likely 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
- White onions
- Soften and turn golden as the main body of the soup, adding sweetness and savoury depth.
- Butter
- Fat that carries the onions to golden browning and adds richness.
- Flour
- Thickens the bouillon into a smooth, body-rich soup when cooked through.
- Bouillon
- Forms the liquid base of the soup, carrying the flavour of the dish.
- Parmesan cheese
- Melts and browns on top, adding a crisp, savoury crust.
The step that matters
Cook the flour into the bouillon until smooth. Raw flour tastes pasty and can leave lumps. Blending it into the fat first and then cooking it in the hot liquid for the full 10 minutes lets the starch swell evenly and removes the raw flour flavour.
“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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