2. In a small mixing bowl, combine the mayonnaise, basil, garlic, salt and pepper. (The mayonnaise can be made in advance and stored in the refrigerator for up to 2 days.)
3. In a large bowl, combine the ground sirloin, sun-dried tomatoes, salt and pepper. Mix well. Using your hands, lightly shape into four 1/2-inch thick patties. (Try to leave some air in the burger and avoid packing them too densely.)
4. Cook the burgers on both sides over the hot grill for about 5 to 6 minutes per side for medium burgers.
5. Lightly toast the buns on the grill.
6. Serve the burgers on toasted buns with basil-mayonnaise and lettuce leaves.