Peel the outer onion skin off. Cut the top off and turn onion over with cut side down. Make vertical slice down the middle.
Using a mandolin: (trim both ends of onion). Turn onion half onto larger sliced side (not the top/bottom sidand slice thin slices with both halves.
Using knife: Turn onion half onto larger sliced side (not the top side and make downward, thin slices to both halves. Discard root end.
Pack slices into a jar or container that can be sealed. Cover with rice wine vinegar. (See Note 1)
Cover and refrigerate a minimum 2 hours or up to a month (if they last!)
Notes
1. ** Add additional flavorings if you prefer like a pinch of red pepper flakes, crushed garlic cloves, sliced jalapeños or herbs. I like to keep them plain most often.