In a small pot, add the vinegar, water, salt, and sugar, and bring to a boil.
Add the hot water/vinegar mix to the jar with the onions, so that all onions are covered. If you need a little more liquid, you can top with hot water.
Place the lid on tightly, let the jar cool, and leave in the fridge overnight.
Recipe Notes
You can store for up to 2 weeks! My jar is empty within days. Enjoy!