Preheat the oven to 400*F. Lightly spray a nonstick baking sheet with vegetable oil spray. Lightly spray both sides of the mushrooms with vegetable oil spray; place with the stem side up on the baking sheet.
Bake for 12 minutes, or until fork tender. Set aside.
Meanwhile, heat a large nonstick skillet over medium-high heat. Remove from heat and lightly spray with vegetable oil spray (being careful not to spray near a gas flame). Cook the zucchini, bell peppers, onion, and basil for 5 minutes, or until the zucchini is tender. Stir in the tomato sauce, sugar, salt, and red pepper flakes; cook for 4 minutes, or until thickened, stirring frequently.
Spoon 1/2 cup zucchini mixture over each mushroom; top with the mozzarella cheese.
Bake for 5 minutes, or until the mozzarella has melted. Remove frome oven. Sprinkle with the Parmesan.