Prepare your jalapeños: Wash the jalapeños thoroughly and remove the stems. Slice them thinly, about 1/8-inch thick. If you prefer less heat, you can remove the seeds and membranes, but keep some for that signature jalapeño bite.
Create the brine: In a small saucepan, combine the white vinegar, water, kosher salt, and sugar. Stir over medium heat until the salt and sugar are completely dissolved. Bring the mixture to a gentle boil.
Assemble the jars: While the brine heats up, place the smashed garlic cloves, black peppercorns, and optional red pepper flakes into a clean 1-quart mason jar or two 1-pint jars. Tightly pack the sliced jalapeños into the jar(s), leaving about 1/2 inch of headspace at the top.
Pour the brine: Carefully pour the hot brine over the jalapeños in the jar(s), ensuring they are fully submerged. Make sure to leave about 1/2 inch of headspace.
Cool and refrigerate: Let the jar(s) cool on the counter for about 30-60 minutes. Once cooled, screw on the lids tightly and transfer the jar(s) to the refrigerator. The jalapeños will be ready to enjoy after at least a few hours, but are best after 24 hours.