Cut the ends off the onions and remove the outer skins. Slice as thinly as possible. 1/8-inch is best unless you like crunchier onion slices then slice them about 1/4 inch or so.Pack the sliced onions in your pint-size mason jars. Depending on how big your onions are, this recipe makes about 2 pints. Sprinkle the peppercorns, the garlic pieces, and the red pepper flakes on top of the sliced onions in the jars.Grab a medium saucepan, and combine the vinegar, sugar, water, and pickling salt and heat over medium heat until the sugar and salt have dissolved. Let it cool for about 20 minutes. Pour the vinegar mixture over the onions in the jars. Please keep them refrigerated in airtight jars. Let them sit in the refrigerator for 30 minutes or overnight. They should be eaten within 7 days. Enjoy.