Slice the red onion thinly.
Bring to boil the vinegar, water, sugar, salt, peppercorns, and optional ingredients in a saucepan until sugar and salt dissolve.
Put sliced onions in a clean jar.
Pour warm brine over onions, ensuring they're submerged.
Let cool, seal, and refrigerate.
After at least 1 hour, pickled onions are ready to use. For the best flavor, wait 2-3 hours or overnight.
