Onion does work very well for bee or wasp stings- simply cut in half, score the onion surface until juicy, and apply directly to skin/sting site.
It does not "draw out" the toxin, rather the onion juice/ phenolic compounds actually break down the proteins of the bee/wasp venom.
My 4 year old had a wasp sting on his back at one point- I held the onion to the site for 10 minutes, and swelling and pain were nearly gone... when I removed the onion, the redness and swelling visibly began to reappear (venom proteins were still present causing the reaction). I re-applied the onion again for 30-45 minutes, and pain and swelling reduced immediately, and permanently after that period of time. It's a great natural remedy... keep some on hand! :)
Slice of onion took the pain away from a bee sting.