Combine the ground beef, sausage, bell peppers, salt and Parmesan and mix together, being careful not to overmix.
Heat grill to medium-high heat. While grill heats, form mixture into 8 patties, using about 2/3 cup of meat mixture for each. Press to 3/4 to 1 inch thick. Grill, turning once, until just cooked through, about 6 min total.
Slice the mozzarella and serve patties with a slice of mozzarella and sliced tomato wrapped in a lettuce leaf. Add basil leaves if desired. Alternately serve patties over a salad with diced mozzarella and tomatoes, drizzled with vinaigrette.