Preheat your oven to 375°F (190°C).
Cook the chicken breasts in a pot of boiling water for 15 minutes or until fully cooked. Once done, shred the chicken using two forks and set aside.
In a skillet over medium heat, sauté the diced onion, garlic, and green bell pepper until they are soft and the onions are translucent.
To the skillet, add the shredded chicken, green chiles, cumin, chili powder, salt, and black pepper. Stir to combine and cook for an additional 2 minutes.
Pour half of the enchilada sauce into the bottom of a 9x13-inch (23x33 cm) baking dish.
Spread the chicken mixture evenly over the sauce in the baking dish.
Drizzle the remaining enchilada sauce over the chicken layer.
Sprinkle the shredded cheese on top, ensuring an even coverage.
Bake in the preheated oven for 20 minutes, or until the cheese is bubbly and slightly golden.
Remove from the oven and let it cool for 5 minutes.
Garnish with fresh cilantro, sour cream, and avocado slices before serving.