Bring vinegar, water, salt and sugar to boil in a small, covered pot over medium-high heat. Stir until the sugar and salt are dissolved. Set aside to cool slightly while you slice the onions.
Thinly slice red onions (using a mandoline if you have one) and place into a 16-ounce heat-proof mason jar or bowl.
Carefully pour the hot vinegar mixture over the onions. Set aside to cool to room temperature, then screw on the lid and store in the refrigerator for up to 2 weeks. OR let sit for 30-60 minutes then add to your salads, sandwiches, tacos, etc.
Notes
They can be stored in a sealed mason jar or other type of airtight container in the fridge for up to 2 weeks. They will not hold up well to freezing.