Cook about 1 1/2 pounds of boneless chicken breasts for 20 to 25 minutes, or use leftover roast or rotisserie chicken. Cooked chicken must reach 165°F internally; cool it slightly, remove skin and bones if needed, and shred enough meat to make 3 cups.

Heat the oven to 350°F and lightly grease a 9-by-13-inch baking dish. This prevents the sauce and tortillas from sticking during baking and makes serving easier.

In a large bowl, stir together the cream of chicken soup, cream of mushroom soup, undrained green chiles, sour cream, milk, chopped onion, salt, pepper, and optional chili powder or cumin. Mix until smooth and evenly combined.

Spread a thin layer of the soup mixture over the bottom of the prepared baking dish. This first layer keeps the tortillas moist and adds sauce beneath the rolled enchiladas.

Warm the corn tortillas briefly in a skillet or microwave. Lightly brush or fry them with the vegetable oil until flexible, taking care not to crisp them; softened tortillas roll without cracking.

Place roughly 1/4 cup of shredded chicken and a small handful of Cheddar down the center of one tortilla. Roll it tightly and set it seam-side down in the baking dish, then repeat with the remaining tortillas, chicken, and cheese.

Pour the remaining soup mixture evenly over the rolled enchiladas. Sprinkle the remaining Cheddar over the top so the casserole develops a golden, bubbling cheese layer.

Cover the dish with foil and bake at 350°F for 20 minutes. Remove the foil and continue baking for 10 to 15 minutes, until the center is hot, the sauce is bubbling, and the cheese is lightly browned.

Let the enchiladas stand for 5 to 10 minutes before serving. This short rest allows the creamy sauce to settle and makes it easier to lift each enchilada from the dish.