I wouldn't recommend putting mint in the ground even contained in a pot - it will go through the bottom of the pot and spread everywhere, and it will certainly do that out of a pot. Spreads by roots underground, so you may plant it in a particular spot, but its likely to appear above ground a foot or more away from the original plant next year and ongoing.

Is there a specific reason for wanting to plant mint among rows of onions? I'm curious, as it seems an odd thing to do.

I'm not finding mint in any list of recommended companion plants for onions, but it's also not listed as harmful--outside of a tendency to turn into a thug! I might keep some mint in a pot nearby, since the flowers attract beneficial insects.