Put up a kettle of water to boil.
Meanwhile, combine the vinegar, water, sweetener-of-choice, salt, and peppercorns in a bowl and stir until sugar is dissolved.
Place the onion slices in a large bowl. When the water boils, pour all of it onto the onions and let them sit for 5 minutes, causing them to wilt slightly. Drain well and transfer the onions to the marinade.
Cover and marinate – at room temperature or in the refrigerator - for at least several hours (but can be days or weeks as well). Store in a tightly closed container in the refrigerator. This keeps indefinitely!