Name: To Pickle Onions
Ingredients: small button onions; strong brine (salt and water); a small bit of alum; cold distilled vinegar; bruised ginger (optional); whole pepper (optional); 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 reduce eye irritation while peeling.
Peel the onions and place them in a strong brine of salt and water with a small bit of alum; let them sit 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 hot brine over them.
Cover the jar tightly and let it sit overnight.
The next day, drain and dry the onions with a cloth, then immerse them in cold distilled vinegar.
Optionally, add bruised ginger, whole pepper, mace, and sliced horseradish to the vinegar for flavor.

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 ginger

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

Allspicefits better
Tarragonfits better
Garlicfits better
Gingerthe original

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