Prepare burgers as recipe directs and refrigerate patties until ready to cook.
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.)
Grill or broil burgers as recipe directs. Top with mozzarella during last minute of cooking time.
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.