Bring 1 1/2 tsp. kosher salt, 1/2 cup apple cider vinegar, 1 Tbsp. sugar, and 1 cup water to a boil in a small saucepan, and then remove from the heat. Thinly slice 1 red onion, stir it into the liquid mixture, and allow to cool. Drain the liquid and that's it—you've got pickled red onions.