Preheat oven to 375 degrees F. In a large skillet cook mushrooms and onion in hot oil over medium heat 6 to 8 minutes or until lightly browned and tender.
In four 10-oz. individual casseroles, layer half of the squash with half of the mushroom mixture. Spoon half of the sauce over layers and sprinkle with Italian seasoning. Repeat layers. Bake, uncovered, about 30 minutes or until squash is tender, topping with cheese the last 5 minutes.
Red Pepper Marinara Sauce
In a large skillet heat oil over medium heat. Add sweet peppers and garlic. Cook and stir about 6 minutes or until tender. Stir in remaining ingredients. Bring to boiling; reduce heat. Simmer, uncovered, about 10 minutes or until thickened and tomatoes are soft. Remove and cool slightly. Blend or process until smooth.