I'm still trying to figure out how people say it without a little bit of g-sound in it. When you say the "un" part of onion, do you say it at the front of your mouth, or the back?
When I say "un" in words like UNDO or UNDERWEAR, I say it at the front of my mouth with my tongue near the roof of my mouth. But when I say "onion" it's near the back of my mouth, and my tongue isn't near the top. The G isn't conscious, it's just a sound that slips in between the "un" and "yun." ;-)