Heat oven to 350°F. Spray bottom of 13x9-inch (3-quart) glass baking dish with cooking spray. Spread 1/2 cup enchilada sauce in bottom of baking dish. In medium bowl, stir together chicken, Monterey Jack cheese, sour cream and chiles.
Spoon about 1/3 cup chicken mixture down center of each tortilla. Roll up tortillas; arrange, seam sides down, in baking dish. Top enchiladas with remaining enchilada sauce. Sprinkle with Cheddar cheese. Spray sheet of foil with cooking spray; cover baking dish with foil, sprayed side down.
Bake 35 minutes. Remove foil; bake 5 to 10 minutes longer or until hot and cheese is melted. Top with avocado or green onions. If desired, serve with lettuce, chopped tomato and additional sour cream.