Start by thinly slicing the red onion.

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

If you're using spices, add them to the saucepan now.

Bring the mixture to a gentle boil.

Remove the saucepan from heat and pour the hot liquid over the sliced onions in a heatproof jar.

Allow the jar to cool at room temperature for about 15 minutes.

Once cooled, seal the jar and refrigerate.

Use the pickled onions as a topping or side for your favorite dishes.