Thinly slice the red onions and prepare all ingredients.

In a medium saucepan, combine vinegar, water, sugar, and salt. Heat the mixture over medium until the sugar and salt dissolve completely.

Pour the pickling liquid over the sliced onions in a glass jar or bowl, ensuring they are submerged.

Let the mixture cool to room temperature.

Cover the container and place it in the fridge. The pickled onions will be ready in about an hour but are best after sitting overnight.

Use the pickled onions as a tangy topping for various dishes.