Peel your red onion and slice it as thinly as possible with an extra sharp knife.
Place slices into a 16 ounce sized canning jar. Add optional rosemary sprig and black peppercorns, if you choose.
In a small saucepan, bring both vinegars to a low boil, adding in sugar and salt. Whisk until they dissolve.
Using a funnel, pour vinegar into jar. Ensure onions are completely covered.
Allow jar to cool to room temperature before putting lid on and storing in refrigerator.