I've got a couple of onions in my indoor bin right now. I cut them up to decompose faster. They don't go away as fast as an apple or something, but there is no odor at all. They will eat it as it decomposes. Throw it in!

Just as an aside, we got a potato plant from the bin that came from kitchen scraps over the winter. Mrs. Joe rescued the sprout from the bin and planted it in a big pot. Its totally happy! (She's Irish, and has an empathy with potatoes.)