Gently combine ground beef, 1/4 teaspoon salt, and 1/4 teaspoon freshly ground black pepper in a bowl. Divide beef mixture into 8 equal portions, shaping each into a 1/4-inch-thick patty. Heat a grill pan over medium-high heat. Coat pan with cooking spray. Add patties to pan; cook for 3 minutes on each side or until desired degree of doneness. Spread the bottom half of each bun with 3/4 teaspoon pesto, and top each with 1 tomato slice and 1 patty. Top each patty with 3/4 teaspoon shaved Parmigiano-Reggiano cheese and the top half of bun.