Start by preheating your oven to 350°F (175°C). Lightly grease a 9×13-inch casserole or baking dish and set it aside. This helps prevent the enchiladas from sticking and makes cleanup easier after baking.
In a large skillet over medium heat, melt the butter. Add the chopped onions and green chiles, then sauté for about 3–4 minutes until the onions are soft and fragrant. Stir in the flour and cook for a minute, then slowly whisk in the chicken broth until the sauce is smooth and slightly thickened. Stir in the sour cream and let it cook for another 2–3 minutes until creamy.
In a large bowl, combine half of the prepared sauce with all of the shredded chicken and half of the shredded mozzarella cheese. This mix becomes the flavorful filling for your enchiladas. The sauce keeps the chicken juicy and gives every bite a creamy finish.
Take one tortilla at a time, add a spoonful of the chicken mixture down the center, then roll it up tightly. Place each rolled tortilla seam-side down into the prepared baking dish. Continue until all tortillas are filled and placed in the dish.
Pour the remaining sauce evenly over the rolled enchiladas. Sprinkle the rest of the mozzarella cheese over the top. Cover the dish with foil and bake for 20 minutes. Then, remove the foil and bake for another 15–20 minutes until the cheese is melted and the sauce is bubbling hot.
Once baked, remove the enchiladas from the oven and let them sit for 5 minutes. Serve warm with your favorite toppings like salsa, cilantro, avocado, or chopped green onions for extra flavor and texture.