Prepare pickled onions: Thinly slice the onions and put into a pickling jar or a small bowl. In a separate microwavable bowl or measuring cup, mix the two vinegars and microwave for two and a half minutes. Meanwhile, toss the garlic, mustard, and salt and pepper in with the onions. Pour the hot vinegar over the onion mixture and allow to sit at room temperature for a few hours. Serve or refrigerate for up to a few weeks.