In a saucepan over medium heat, combine 1 cup vinegar, 1/2 cup water, 1/2 teaspoon salt. Bring to a light boil or until all of the salt is dissolved.
Chop up a red onion. Cut it into quarters and then slice lengthwise.
Give the slices a quick toss and then stuff them in a pint-sized glass jar. Add the vinegar mixture along with 1 garlic clove chopped in half and some freshly ground pepper (or whole peppercorns).
Let cool on counter. Then cover and store in the refrigerator.
The pickled onions will have most of their flavor after a few hours of resting in the brine.
These pickled onions will keep in the fridge for at least a few weeks.
I don't add sugar to this recipe, but a standard amount for a pint-sized jar of pickled onions would be 1 tablespoon.
Apple cider vinegar is also commonly used for pickling and is a viable option here.