1.Chop half onion finely; cut remaining half into four slices.
2.In a medium bowl, combine mince, 1 tbsp barbecue sauce, tomato sauce and chopped onion. Shape mixture into four patties.
3.On a heated, oiled grill plate, cook pineapple slices and onion slices until pineapple is browned. Cook patties on same grill plate.
4.Preheat grill. Split buns; toast, cut-side up, under grill.
5.Spread remaining barbecue sauce on bun bases; layer ingredients between bun halves.