Preheat oven to 220°c/200°c fan-forced. Line a baking tray with baking paper.
Place mince, onion, sauce, garlic powder, and egg in a bowl. Season, then mix well to combine. Shape mixture into 4 patties. Fold 4 cheese slices into eighths, then carefully press into centres of patties, molding mince to enclose the cheese. Chill patties in the fridge for 15 minutes or until firm.
Meanwhile, place bacon on a tray. Brush both sides with syrup, then bake for 15 minutes, turning once halfway, or until golden and crisp.
Meanwhile, heat oil in a large frying pan over medium heat. Cook patties for 5 minutes on each side, top each with remaining cheese in the last 2 minutes of cooking, or until cooked through.
Spread bun bases with extra sauce, then top with lettuce, tomato, bacon, and patties. Sandwich with roll tops and serve.