In a small saucepan on medium heat, add white vinegar, apple cider vinegar, water, sugar and salt. Whisk until the salt and sugar dissolves. Mixture should be hot but not boiling by the time the sugar is dissolved, about 3 minutes. Remove from heat and let the mixture cool for 5 minutes.
In a 32 ounce mason jar, add the garlic, peppercorns and sliced onions. Put the vinegar mixture into the mason jar. Depending on how much onion is in your jar, you may have left over vinegar that you can discard. If you do not have enough liquid to fill the jar to the top, add water until it reaches the top of the jar.
Let the mixture continue to cool for about 30 minutes before covering the jar and place in the fridge. Ready to eat when the onions turn pink, after at least 1 hour in the fridge. The onions will last about 2 weeks in the fridge. Enjoy on salads or sandwiches.