Step 1

Trim both ends of scallions and cut in half crosswise. Pack into a 1-pint heatproof jar.

Step 2

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.

Step 3

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.

Step 4

Do Ahead: Scallions can be made 5 days ahead. Cover and chill.