Preheat oven to 350°F. Coat an unheated large nonstick skillet with cooking spray. Preheat skillet over medium-high heat. In a medium bowl, toss together chicken, garlic, and chili powder. Add to hot skillet. Cook for 4 to 6 minutes or until chicken is no longer pink, stirring frequently. Remove chicken from skillet; set aside.
Pour oil into hot skillet. Add onion and sweet pepper. Cook over medium heat about 5 minutes or until tender, stirring occasionally. Stir in spinach.
Coat a 2-quart square baking dish with cooking spray. Spread about 1/2 cup of the salsa into the bottom of the baking dish. Top with half of the tortilla pieces, half of the chicken mixture, and half of the vegetable mixture. Pour half of the remaining salsa over the vegetables and top with half of the cheese. Repeat layers once, except do not top with the remaining cheese.
Bake, covered with foil, for 30 to 35 minutes or until heated through. Sprinkle with the remaining cheese. Let stand for 5 minutes before serving. If desired, garnish with cherry tomatoes, chopped avocado, and/or fresh cilantro.