I use an onion holder when I need to slice one.

There are a lot of models out, but I recommend one of the stainless ones as the the wooden ones seem to lose their tines too easily.

Hmm, can't post a link or an image till I have 10 or more posts, but a search for "onion holder" will come up with quite a few if anyone is interested.

Makes slicing round things much safer.

Better hurry up and post some more.