I pickle mine in malt vinegar sliced very thinkly red and white, and a splash of balsamic too, some pepper and after 3/4 days they are lovely in salads and everything. If your talking silverskin or such like, not too sure how to do them, although wouldn't mind some tips - I love onions of all shapes and sized, and expecially pickled with some cheese YUM!
I should have said I add salt to get them really crunchy and a really strong onion, you could even use shallots instead of onions if you want them reall really strong.
Put the unskinned onions in enough brine to cover (4oz salt to 1pt water) for 24 hrs. Skin them & put them in fresh brine for 24-36 hours. Rinse well & pat dry before bottling. I`ve tried Schwartz pickling spice, but go easy with it as it can make for a fiery onion! So I don`t put salt in with the vinegar. Do sterilize jars first.