Using a mandoline, or a very sharp knife, thinly slice the onions into ⅛ inch thick slices. Divide the onions evenly between two pint-size jars or one quart-size jar.
If using, add ½ teaspoon of peppercorns to each jar of onions.
In a medium saucepan, heat the water, vinegar, salt, and sugar over medium heat, until the sugar and salt are dissolved, whisking occasionally. Remove from the heat and allow to cool slightly about 5 minutes.
Pour the vinegar mixture over the onions, dividing the mixture evenly between the jars. Use a fork or spoon to submerge the onions fully in the vinegar mixture.
Place the lid on the jars and refrigerate for at least 30 minutes before serving.
Notes
Storage: Pickled Red Onions will last in the refrigerator between 2-3 weeks. Onions: While red onions are the standard choice for pickling, feel free to use any onion you like. Keep in mind if your onions are not cut thin, it will take longer for them to become pickled. Vinegar: Use white distilled vinegar, apple cider vinegar, or rice wine vinegar. Sugar: Feel free to use honey, maple syrup, or granulated sugar. If you like really sweet pickled onions, double the amount of sweetener used.