If going for the sweeter route, roasted garlic and roasted or carmelized onions totally work! I'd not go with fresh garlic or onions as it is too strong/sharp for the flavor it sounds like you are looking for. But using roasted/carmelized garlic and onion...with restraint.....Oh YEA!
Sounds yummy. Would you use white or yellow onions... Im thinking yellows would be sweeter