First, thinly slice your red onion using a mandoline slicer. Next, mix your vinegars and water, heat in a bowl or tea kettle. While the liquid is heating, pack your jar (or jars) a little more than 3/4 full with onion slices. Top with salt and sugar. Next, once your liquid is at least near-boiling (boiling is fine too!), fill the jar and leave 1/4 inch headspace. Stir very well. You can also put on the lid and shake the jar rigorously. Set aside for 30 minutes, and then onions will be ready! Refrigerate once cool and consume within 5 days.