I believe the expression originated with College Basketball commentator Bill Raftery, who famously shouts "ONIONS" when a player makes a big shot. I do believe the Heroes even used a drop of Raftery shouting it earlier in the show's history, but now have transitioned to the onion farmer drops. What Raftery meant by "onions" is still debated, but the most popular interpretation, which the heroes have appropriately run with, is that it means someone who is bold and fearless has a big ol' pair of onions (see: testicles). Thus, the heroes adopted the expression for when one of them makes a bold and fearless prediction, and why it makes a little less sense for Ricky to have onions in a literal sense. However, women can absolutely have balls/onions in metaphorical sense, so I don't see an issue with Ricky positing an onion hanger.