Rinse out two 8 oz glass jars or one 16 oz glass jar with lids. Prepare the onions by using a mandoline or a very sharp knife to slice them into thin circles. Place the onions in the jars. You can pack them in pretty tightly.
In a small saucepan, combine the vinegars, water, sugar, and salt. Bring to a boil and remove from heat.
Pour the brine over the onions (half in each 8 oz jar or the whole thing in the 16 oz jar). Cover the jars with their lids and let the onions sit for at least 30 minutes at room temperature, and up to an hour.
Enjoy right away or store in the fridge for later use. They will keep for about 2 to 3 weeks!
Adding sugar helps to even out the vinegar taste. You can add up to 2 tbsp if you would like them to be sweeter.