Preheat oven to 350°F (175°C) and grease a 9x13-inch baking dish with butter or cooking spray.

In a large saucepan over medium heat, melt butter. Add flour and whisk constantly for 2-3 minutes until golden to create a roux.

Gradually pour chicken broth into the roux while whisking continuously. Continue whisking as the mixture thickens for about 5 minutes.

Reduce heat to low and stir in sour cream, garlic powder, cumin, salt, and black pepper. Whisk until smooth and remove from heat. Set aside 1 cup of white sauce.

In a mixing bowl, combine shredded chicken, green chilies, 1 cup monterey jack cheese, 1/2 cup cheddar cheese, chopped cilantro, and the reserved 1 cup of white sauce.

Warm tortillas in microwave for 30 seconds wrapped in damp paper towel or heat in dry skillet for 10 seconds per side.

Place 1/3 cup chicken mixture down center of each tortilla. Roll tightly and place seam-side down in prepared baking dish.

Pour remaining white sauce evenly over all enchiladas. Sprinkle remaining monterey jack and cheddar cheese over the top.

Cover with aluminum foil and bake for 20 minutes. Remove foil and bake 10-15 minutes more until cheese is melted and golden.

Let rest 5 minutes. Garnish with diced tomatoes and fresh cilantro before serving.