Dredge chicken in flour, salt and pepper and brown on both sides, until breasts are cooked through and no longer pink. Remove from pan and set aside. In same pan, saute onion, zucchini, eggplant and mushrooms until tender. Add chicken back to pan and pour spaghetti sauce over all. Add 1-2 slices of mozzarella on top of each breast, cover pan and heat until sauce is hot and cheese is melted.