I found a recipe for pickling them (and if you haven't realized I really do love just about everything pickled) at Closet Cooking here.
Because I had sooo much onion I chopped them up and put them in a half-gallon jar (just like I did to the nectarines, I guess it is a lucky thing that I have so many empty kimchi jars) and I quadrupled the recipe.
Both jars are cooling down on my counter right now, waiting to go into the fridge.