Cook the Chicken

1. Heat the oven to 425°F / 220°C with a rack in the middle. Pat the chicken dry, then toss it with 1 tbsp / 15 ml oil, 1 tsp / 6 g salt, and the black pepper.
2. Arrange the chicken on a small sheet pan or baking dish in a single layer. Roast for 20 to 24 minutes, until the thickest piece reaches at least 175°F / 79°C for easy shredding, or at least 165°F / 74°C at minimum.
3. Let the chicken rest for 10 minutes, then shred it into bite-size pieces. You want moist strands, not finely chopped bits.

Make the Sauce

1. Put the guajillo and ancho chiles in a bowl and cover them with very hot water. Let them soak for 10 minutes, until softened and flexible.
2. While the chiles soak, finely chop ½ of the onion and set it aside for the filling. Roughly chop the remaining ½ onion for the sauce.
3. Heat 1 tbsp / 15 ml oil in a medium saucepan over medium heat. Add the roughly chopped onion and ½ tsp / 3 g salt, and cook for 4 to 5 minutes, until softened. Add the garlic, tomato paste, cumin, and oregano, and stir for 30 seconds.
4. Drain the chiles. Blend the softened chiles, the onion mixture, and the chicken broth until very smooth. Pour the sauce back into the saucepan and simmer for 10 to 12 minutes, until slightly thickened and glossy.

Fill and Bake

1. Lower the oven to 375°F / 190°C. Spread ½ cup / 120 ml sauce over the bottom of the 9 x 13-inch / 23 x 33-cm baking dish.
2. In a mixing bowl, combine the shredded chicken, the finely chopped onion, the cilantro, 4 oz / 115 g of the cheese, and ½ cup / 120 ml of the sauce. Mix until the filling looks lightly coated and evenly combined.
3. Brush the tortillas lightly on both sides with the remaining 1 tbsp / 15 ml oil. Stack them, cover, and microwave for 45 to 60 seconds, until warm and flexible.
4. Fill each tortilla with a scant ¼ cup of the chicken mixture, roll it snugly, and place it seam-side down in the dish. Arrange all 12 in a single layer.
5. Spoon the remaining sauce over the enchiladas, then scatter the remaining 6 oz / 170 g cheese over the top. Bake for 20 to 25 minutes, until the edges are bubbling, the cheese is fully melted, and the centers are hot.
6. Let the pan rest for 10 minutes before serving. Success looks like tender tortillas that still hold together, glossy sauce that clings instead of pooling thinly, and a filling that stays juicy all the way through.