Peel and slice the red onion into thin slices. Pack onions into a pint sized Mason jar.
In a small saucepan stir together vinegar, sugar, and salt. Add in 1 to 2 flavorings if desired. Bring to a boil; remove from heat.
Carefully, pour the hot liquid over the onions in the jar leaving 1/4-inch headspace. Wipe the rim clean. Place the lid on the jar and screw on a band fingertip tight. Allow to cool to room temperature.
Refrigerate for up to 3 weeks, but best if eaten in the first week.
Cook’s Note: The trick to making truly quick pickled onions is to slice the onions very thin (about 1/8-inch) so they soften up and absorb the vinegar quickly. You can do this with a sharp chef’s knife or a mandoline.
Slice the onions thicker (about 1/4-inch) for more of a crunch, but be aware you will need a few hours or overnight to give the vinegar time to pickle the onions.