Peel off the outer onion skin; cut the top off and turn onion over with cut side down; discard root end.
Make vertical slices down the middle.
Using a knife, turn onion half onto larger sliced side and make downward, thin slices to both halves.
Pack slices into a jar or container that can be sealed.
Cover with rice wine vinegar.
Cover and refrigerate a minimum 2 hours or up to a month.