Step 1: Preheat your oven to 375°F (190°C). Lightly grease a 9×13-inch baking dish with cooking spray.
Step 2: In a large bowl, combine the shredded chicken, half of the chopped onion, 1 cup of the shredded cheese, and 1/2 cup of the enchilada sauce. Add the diced green chiles (if using) and season with salt and pepper. Mix well until everything is evenly combined.
Step 3: Spread about 1/2 cup of enchilada sauce evenly on the bottom of the prepared baking dish.
Step 4: To soften tortillas (crucial for easy rolling): You can either quickly dip each tortilla into the warm enchilada sauce, or lightly pan-fry each tortilla in a thin layer of olive oil for 15-20 seconds per side until pliable, or wrap a stack of 4-5 tortillas in a damp paper towel and microwave for 30-60 seconds.
Step 5: Lay a softened tortilla on a clean surface. Spoon about 1/4 to 1/3 cup of the chicken mixture down the center of the tortilla. Sprinkle with a little extra cheese from the remaining 1 cup.
Step 6: Tightly roll up the tortilla and place it seam-down in the prepared baking dish. Repeat with the remaining tortillas and chicken mixture, arranging them snugly in a single layer.
Step 7: Pour the remaining enchilada sauce evenly over the rolled enchiladas, ensuring they are well-covered.
Step 8: Generously sprinkle the remaining shredded cheese over the top of the enchiladas.
Step 9: Bake for 20-25 minutes, or until the cheese is melted and bubbly, and the sauce is simmering around the edges. If desired, broil for 1-2 minutes at the end for a golden-brown top (watch carefully to prevent burning).
Step 10: Remove from the oven and let rest for 5-10 minutes before serving. This helps the enchiladas set and makes them easier to serve. Garnish with optional toppings like sour cream, fresh cilantro, or diced avocado.