Place 3/4 cup white wine or cider vinegar, 2 Tbsp. granulated sugar, 1/2 tsp. salt, 12 black peppercorns and pinch red pepper flakes in a small pot. Bring to boil, cook and whisk 30 seconds to dissolve the sugar. Pour hot vinegar mixture over the onions. Press on onions to submerge them into the liquid, if necessary. Cool onions to room temperature, and cover and refrigerate 4 hours, or preferably overnight, before serving. The onions will keep about a week in the refrigerator.