Preheat oven to 350°F. Spray 13x9-inch baking dish with cooking spray; set aside. Heat oil in medium saucepan over medium heat. Add onion and garlic; cook and stir 5 minutes or until onion is crisp-tender.
Stir in tomato puree, chili powder, salt and pepper. Reduce heat to low; simmer 10 minutes, stirring occasionally.
Layer 4 tortillas, half of the chicken, 2/3 cup cheese and 1/4 of the sauce in prepared dish. Repeat layers. Top with remaining 4 tortillas, remaining sauce and remaining 2/3 cup cheese. Cover with aluminum foil.
Bake 30 minutes. Uncover; bake 5 minutes more. Cut into 8 pieces.