In a large ovenproof skillet, saute onion in 1 tablespoon oil for 3 minutes or until crisp-tender. Add garlic; cook 1 minute longer. Add zucchini and red pepper; saute for 3 minutes or until crisp-tender.
Remove from the heat; stir in the cheese, basil, salt and pepper. Carefully loosen the skin on one side of each chicken breast to form a pocket; stuff with vegetable mixture.
In the same skillet, brown chicken skin side down in remaining oil. Turn chicken. Bake, uncovered, at 375° for 25-30 minutes or until a thermometer reads 170°.