Was just showing the hubby the secret to my stock this weekend -- and that is I stick a couple of whole cloves into the onion chunks. Sometimes maybe I have to start with a knife tip, but it helps hold the skins on to the onions. So, if I use a whole onion, maybe I'll have up to 10-12 cloves for a pot of beef stock, and probably half that for chicken.
It adds a richness you can't quite place but you can notice when it's not there.