Raw onions affect a lot of people. I too am affected by the sulphur compounds found in the allium family. Gives me a belly ache but not necessarily nausea. That goes for almost all the onion family from chives, green onions, on up to large sweet mild onions. Cooked is a different matter and I almost always cook yellow onions, shallot or scallions to go along with... everything from eggs to steaks!
I do not recommend eating just onion alone. It is very hard on your stomach. If you must consume raw onion, soaking in water for 15 20 minutes before consuming might help rid of some of the sulphur. But personally I dont' think it will stop your gut ache enough to warrant even thinking about it.
So now I must ask, why are you eating raw onions alone?