Slice 2–3 red onions into thin rings or half-moons
Separate into layers and place in a heat-safe jar or bowl
In a saucepan, combine 1 cup white vinegar (or apple cider vinegar), 1 cup water, 1/4 cup sugar, and 1 1/2 teaspoons kosher salt
Heat until the sugar and salt dissolve
Optional: add 1–2 crushed garlic cloves, 1 teaspoon whole black peppercorns, 1–2 bay leaves, and/or 1 teaspoon mustard seeds to the jar
Pour hot brine over the onions until fully submerged
Cool to room temperature
Cover and refrigerate
Let stand at least 30 minutes for mild flavor, or 24 hours for best flavor
Keep refrigerated and use within 2–4 weeks