1/4 cup minced onion
3 tablespoons minced dill pickle
2 tablespoons pickle brine
2 tablespoons distilled white vinegar
1 tablespoon prepared white horseradish
1 tablespoon sugar
1 teaspoon kosher salt
1/2 teaspoon celery seeds
1/2 teaspoon freshly ground black pepper
Whisk all ingredients in a medium bowl to blend. DO AHEAD: Can be made 1 day ahead. Keep chilled.