Heat the oven to 350 degrees. Spray a 9-inch square baking dish with cooking spray. Mix the dip mix and sour cream in medium bowl. Whisk in the chicken stock. Pour 1/2 cup of the sour cream mixture in the bottom of the baking dish. Layer 4 tortillas over the sauce in the dish, overlapping. Top the tortillas with 1/2 of the chicken. Spoon 1/3 of the tomatoes with the green chilies over the chicken. Sprinkle the tomatoes with 1/3 of the cheese. Repeat the layers and top with the remaining tortillas, tomatoes and green chilies. Pour the remaining sour cream mixture over the top of the casserole. Let the casserole stand for 5 minutes. Sprinkle the casserole with the remaining cheese. Bake for 35 to 40 minutes or until bubbly and cheese is melted and browned. Let the casserole stand for 5 minutes before serving to make slicing easier.