Try these anywhere you’d use pickled onion, such as in grain bowls, on roasted carrots, or on a cheese sandwich.
- 2 teaspoons mustard seeds
- 1 teaspoon coriander seeds
Trim both ends of scallions and cut in half crosswise. Pack into a 1-pint heatproof jar.
Toast mustard, coriander, and cumin seeds in a dry small saucepan over medium-low heat, tossing often, until fragrant, about 2 minutes. Add to jar.
Bring vinegar, sugar, and salt to a simmer in same saucepan over medium heat, stirring to dissolve sugar and salt. Pour brine over scallions and seal jar. Chill at least 1 day before using.
Do Ahead: Scallions can be made 5 days ahead. Cover and chill.