Prep the Green Tomatoes: Wash the tomatoes thoroughly under cold running water and pat them dry with a clean kitchen towel.
Prepare the Pickling Liquid: In a medium saucepan, combine white vinegar, water, granulated sugar, kosher salt, garlic cloves, black peppercorns, mustard seeds, and crushed red pepper flakes.
Pack the Jars: While the pickling liquid is simmering, pack the prepped tomatoes tightly into a clean, sterilized, 1 (32-ounce) wide-mouth mason jar or smaller mason jars or airtight containers.