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Spanish Onions Au Gratin

A baked dish of boiled Spanish onions topped with breadcrumbs, cheese, butter, and milk.

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 ↓

Spanish Onions Au Gratin

Ingredients

Directions

The book gives no oven temperature. The 350°F (175°C) in the steps was added when the recipe was transcribed, as a starting point.

Prepare

  1. Boil the onions in water, discarding the water after it first boils.

  2. Butter a fireproof dish and arrange the boiled onions in it.

Assemble and bake

  1. Fill the crevices with breadcrumbs and cheese, moisten with milk, and season well.

  2. Top with a layer of breadcrumbs and small dabs of butter.

  3. Bake at 350°F (175°C) until golden brown.

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 + cheesetop 8% of all pairs
    Smell alikemoderate
    Cooked together7,962 recipes

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

  3. Cheese + milktop 10% of all pairs
    Smell alikemoderate
    Cooked together4,426 recipes

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

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

    Shared aroma: decanal (soap, orange peel).

  5. Onion + cheesetop 16% of all pairs
    Smell alikemoderate
    Cooked together4,849 recipes

    Shared aroma: 2-isopropyl-3-methoxypyrazine (earthy, pea).

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

What each ingredient does

Spanish onions
Form the main body of the gratin and become soft and sweet as they cook.
Breadcrumbs
Absorb moisture and form a crisp, browned topping.
Butter
Adds fat for richness and helps the top brown.
Grated cheese
Melts into the crumbs and adds savoury flavour and browning.
Milk
Moistens the crumbs and cheese so the filling stays creamy while baking.

The step that matters

Boiling the onions and discarding the water. Boiling first softens the onions and draws out some of their sharp sulphur compounds, so the gratin tastes milder and cooks more evenly.

Swap the onion

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

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

Fit is the average pairing score against butter, cheese, milk. 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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