Preheat oven to 350°. Cut and discard tops from red peppers; remove seeds. In a 6-qt. stockpot, cook peppers in boiling water until crisp-tender, 3-5 minutes; drain and rinse in cold water., In a large skillet, cook beef over medium heat until no longer pink, breaking into crumbles, 6-8 minutes. Remove with a slotted spoon; pour off drippings., In same pan, heat oil over medium heat; saute zucchini, yellow squash, onion and green pepper until tender, 4-5 minutes. Add spinach and garlic; cook and stir until spinach is wilted, about 1 minute. Stir in cooked beef, rice, tomato sauce, mozzarella cheese and salt., Place red peppers in a greased 8-in. square baking dish. Fill with meat mixture. Bake, covered, until peppers are tender, 35-40 minutes. Top with provolone cheese; bake, uncovered, until cheese is melted, about 5 minutes.