In a large bowl, combine the chicken, chili powder, garlic salt and cumin. Place 1/3 cup chicken mixture down the center of each tortilla. Roll up and place seam side down in two 13x9-in. baking dishes coated with cooking spray.
In a large saucepan, whisk flour and broth until smooth. Bring to a boil; cook and stir for 2 minutes. Reduce heat; stir in sour cream and Monterey Jack cheese until melted.
Pour sauce over enchiladas; sprinkle with Mexican cheese blend.
Bake, uncovered, at 350° until bubbly, 15-20 minutes. Top with tomatoes if desired.