Spray broiler pan with nonstick cooking spray. Sprinkle chicken with garlic powder, 1/4 teaspoon salt and 1/8 teaspoon pepper; place on sprayed broiler pan. Broil 4 to 6 inches from heat for 6 to 8 minutes on each side or until fork-tender and juices run clear.
Meanwhile, heat oil in large nonstick skillet over medium-high heat until hot. Add mushrooms, garlic and 1/4 teaspoon salt; cook 1 to 2 minutes or until garlic is tender, stirring occasionally. Add onion, chopped basil, tomatoes, vinegar and 1/8 teaspoon pepper; cook 30 to 45 seconds or until thoroughly heated.
To serve, arrange chicken on individual plates. Sprinkle with half of cheese. Top each serving with mushroom mixture; sprinkle with remaining cheese. Garnish with basil sprigs.