I am going to make a meatball sandwich for the first time ever. I have these caramelized onion flavored chicken meatballs, some sandwich bread, and some Monterey cheese. What else should go in the sandwich? Moar caramelized onions flavored with angostura? Some other tasty thing that I might not have on hand?

I need to eat the meatballs and the sandwich bread, so those two ingredients are fixed.

Thanks.

Eta: the meatballs are not in any kind of sauce, but I can acquire anything that Trader Joe's carries