1. Prepare burgers as recipe directs and refrigerate patties until ready to cook.
2. Place pine nuts and garlic in a food processor. Add oil; pulse to chop. Add 4 cups basil, Parmesan and salt. Process until finely chopped, stopping and scraping sides of bowl once or twice. (Makes 3/4 cup pesto.)
3. Grill or broil burgers as recipe directs. Top with mozzarella during last minute of cooking time.
4. Spread buns with pesto. Place burgers on buns and serve topped with a slice of tomato and a large basil leaf and assorted toppings, if desired.