1 cup unseasoned rice vinegar
1 1/2 teaspoons coriander seeds
1 1/2 teaspoons sugar
1 1/2 teaspoons kosher salt
1/2 teaspoon peppercorns
7 ounces red Fresno chiles, thinly sliced
Bring vinegar, coriander seeds, sugar, salt, and peppercorns to a boil in a medium saucepan. Stir to dissolve sugar, remove from heat, and add chiles.
Pour into a pint canning jar, cover, and let cool. Chill at least 12 hours and up to 5 days.