1. In a small saucepan, stir together the vinegar, water, salt and desired amount of sweetener. Cook over medium-high heat until the mixture reaches a simmer. (Or alternately, heat the mixture in the microwave.)
2. Place the thinly-sliced onions in a mason jar. Pour the hot vinegar mixture over the onions, screw on the lid, and shake the onions briefly until they are evenly coated with the vinegar mixture.
3. Let the onions marinate for 30 minutes. (You may need to press the onions down with a spoon so that they are all submerged under the vinegar mixture.)
4. Serve immediately, or refrigerate the onions in a sealed container for up to 2 weeks.

Variations:

• Make it spicy with sliced fresh habañero peppers. For for a milder dose of heat, add in a slice or two of jalapeño peppers, or a pinch of crushed red pepper flakes.
• Make it garlicky with a clove or two of thinly sliced garlic.
• Make it peppery with a few whole peppercorns or a few twists of freshly cracked black pepper.