Heat oven to 350°F. Spray 8-inch square (2-quart) glass baking dish with cooking spray. Heat 8-inch nonstick skillet over medium-high heat. Add chicken; cook 2 to 3 minutes, stirring frequently, until lightly browned.
Stir in onion, salt, oregano and pepper. Cook 3 to 5 minutes, stirring frequently, until chicken is no longer pink in center. Cool 5 minutes.
Meanwhile, spread 1/4 cup of the enchilada sauce in baking dish.
Stir 1/2 cup of the cheese, the sour cream and chiles into chicken mixture. Spoon mixture evenly down center of each tortilla; roll up and place seam side down over sauce in dish. Spoon remaining 1/4 cup enchilada sauce over tortillas. Cover with foil.
Bake 30 to 40 minutes or until thoroughly heated. Uncover; sprinkle remaining 1/2 cup cheese over top. Bake uncovered 4 to 5 minutes longer or until cheese is melted. Sprinkle individual servings with lettuce and tomato.