In a pot, combine the vinegar, sugar, salt, and water together and bring to a boil. Cook for 5 minutes.
Divide the sliced onions between 2 clean 12-ounce jars.
Pour the pickling mix into the jars all the way to the top, then add in the peppercorns and seal the lid. Keep in the refrigerator for at least 2 days for the onions to fully pickle.