1 large red onion, thinly sliced
1/2 cup apple cider vinegar
1/2 cup water
1/2 tablespoon kosher salt
1 teaspoon granulated sugar
In a small sealable jar (make sure it’s large enough to fit all of the ingredients), add the sliced red onions.
Add the vinegar, salt, and sugar.
Tightly seal the jar, shake, and let sit in the refrigerator for at least 4 hours, up to two weeks.