Step 1: Prepare the Chicken. Start by preheating your oven to 350°F (175°C). In a large pot, boil the chicken breasts until they are fully cooked, usually about 20-25 minutes. Once cooked, shred the chicken using two forks and set it aside.
Step 2: Mix the Ingredients. In a large mixing bowl, combine the shredded chicken, salt, black pepper, 4 oz of the Mexican cheese, sour cream, and the diced green chilis along with their juice. Stir until everything is well combined.
Step 3: Assemble the Casserole. Pour the mixture into the 9x13 inch baking dish, spreading it out evenly. Top it with the remaining 4 oz of shredded Mexican cheese.
Step 4: Bake. Cover the baking dish with aluminum foil and bake in the preheated oven for 20-25 minutes. Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden.
Step 5: Garnish and Serve. Once out of the oven, let the casserole cool for a few minutes. Garnish with diced avocado, roma tomato, and chopped cilantro before serving. Enjoy your Mexican Chicken Casserole hot!