Whisk ½ cup apple cider vinegar, 1 Tbsp. granulated sugar, 1½ tsp. Diamond Crystal or ¾ tsp. Morton kosher salt, and 1 cup water in a small bowl until sugar and salt dissolve.
Place 1 red onion, thinly sliced, in a mason jar with a tight-fitting lid; pour vinegar mixture over. Let sit at room temperature for 1 hour.
Note: Pickled red onions can be made 2 weeks ahead. Cover and chill. Drain onions before using.