Preheat oven to 350°.
Heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Add onion; cook 5 minutes or until tender, stirring occasionally. Stir in chicken, garlic, chiles, and tomatoes. Reduce heat, and simmer 7 minutes or until liquid evaporates. Set aside.
Combine milk and flour in a small saucepan over medium-high heat; cook 5 minutes or until mixture thickens, stirring constantly with a whisk. Stir in salt.
Warm tortillas according to package directions. Spoon about 1/2 cup chicken mixture and about 2 1/2 tablespoons cheese down center of each tortilla; roll up. Arrange filled tortillas in the bottom of a 13 x 9-inch baking dish coated with cooking spray. Pour milk mixture over tortillas, and top evenly with remaining 1 cup cheese. Bake at 350° for 25 minutes or until cheese is bubbly. Remove from oven.
Broil casserole for 3 minutes or until cheese begins to brown.