Preheat oven to 375°F. Spray a large skillet with nonstick cooking spray. Sauté onion for 5 minutes, stirring occasionally. Add bell pepper and zucchini, cook for 5 minutes more. Stir in chicken; set aside.
Meanwhile, combine enchilada sauce and tomato sauce in a small bowl; add 1⁄2 cup to vegetable and chicken mixture.
Soften tortillas on the stovetop or in the microwave oven. Dip each tortilla in sauce and place equal amounts of vegetable and chicken mixture on one side. Roll up and place in a 13 x 9 inch baking pan.
Pour any remaining sauce over the top. Cover loosely with foil and bake for 20 to 25 minutes. Remove cover and sprinkle cheese over top; bake for 5 minutes more.