Name: To Pickle Onions
Ingredients: small button onions; strong brine (salt and water); a small bit of alum; cold distilled vinegar; bruised ginger; whole pepper; mace (optional); sliced horseradish (optional)

Why it works: Onion and horseradish match in chemistry and in kitchens. They rank in the top 7% of all ingredient pairs, and cooks put them together in 1,784 recipes. Both carry coumarin, which smells sweet, hay. The science ↓

To Pickle Onions

Ingredients

Directions

Choose small button onions of similar size and soak them in warm water to ease peeling.
Peel the onions and place them in a strong brine with a bit of alum overnight.
The next day, boil the onions in the brine for 1 minute.
Alternatively, soak peeled onions in a mixture of milk and water, then drain.
Place the drained onions in a jar and pour boiling brine over them.
Cover the jar and let it sit until the next day, then drain and dry the onions.
Submerge the onions in cold distilled vinegar with bruised ginger, whole pepper, and optional mace and horseradish.
Ensure the onions are always covered with vinegar, cork the jar tightly, and store in a cool, dry place.

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.

Onion + horseradishtop 7% of all pairs

Smell alikestrong

Cooked together1,784 recipes

Shared aroma: coumarin (sweet, hay).
Ginger + peppertop 9% of all pairs

Smell alikestrong

Cooked together6,800 recipes

Shared aroma: 2-isopropyl-3-methoxypyrazine (earthy, pea).
Onion + gingertop 9% of all pairs

Smell alikemoderate

Cooked together7,595 recipes

Shared aroma: 2-isopropyl-3-methoxypyrazine (earthy, pea).
Pepper + horseradishtop 11% of all pairs

Smell alikemoderate

Cooked together2,646 recipes

Shared aroma: coumarin (sweet, hay).
Onion + peppertop 11% of all pairs

Smell alikemoderate

Cooked together68,474 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

Swap the mace

The recipe marks mace as optional. How well each one fits the dish's main ingredients (onion, vinegar, ginger, pepper, horseradish):

Garlicfits better
Turmericfits better
Allspicefits better
Macethe original

Fit is the average pairing score against onion, vinegar, ginger, pepper, horseradish. Candidates are from the same food group and used in at least 100 recipes.