Sterilize a mason jar and set aside. Bring a kettle of water to a boil and while you are waiting, slice the onions on a mandolin or thinly slice and place in a colander.

Place remaining ingredients in the jar and put lid on and shake to dissolve sugar and salt.

When water comes to a boil, rinse the onions over the sink with the boiling water and drain well. Add onions to mason jar and submerge the onions in the brine. Allow to sit on counter until the onions are room temperature and place lid on and keep in refrigerator overnight for best flavor. The onions are ready in 30 minutes, but are better the next day. They will keep in the refrigerator for several weeks.