Heat a saucepan of water over high heat until boiling, then remove from heat.
Add onions and let cool in the water for ten minutes. Drain.
In a non-reactive mixing bowl combine all of the ingredients and mix well. Stuff into a glass jar, gently packing them down. Add just enough additional water to ensure the onions are covered. Seal with a lid, gently shake back and forth to mix if you had to add additional water, then put into the refrigerator overnight.