Using a mandolin or sharp chef's knife, slice the peeled onion into very thin rounds (about 1/8-inch thick). Add the onions to a 12-16 oz. mason jar with the pickling spice layered in with them.
In a small saucepan over medium heat, warm the water, vinegar, sweetener, and salt just until the salt dissolves.
Pour the liquid over the onions. Cover and rest for 20-30 minutes before serving. Store in the refrigerator.