Coat a small nonstick skillet with cooking spray; place over medium heat until hot. Add carrot and onion; sauté 3 minutes. Add spinach; sauté 1 minute. Stir in Italian seasoning and tomato sauce; simmer over medium-low heat 8 minutes or until thickened.
Place muffin halves on a baking sheet. Spread 1 1/2 tablespoons sauce over each, and top with bell pepper and cheese. Bake at 400° for 8 minutes or until cheese melts.