Cook the chicken: Heat a large skillet over medium heat and spray with cooking spray or add a small amount of oil. Add the chicken breast and cook for 4-5 minutes per side until fully cooked (internal temperature of 165°F). Remove from the pan, let it rest for a few minutes, then shred or dice.
Sauté the vegetables: In the same skillet, add the diced onion and bell peppers. Cook for 4-5 minutes, stirring occasionally, until softened.
Combine with sauce and tortillas: Return the shredded chicken to the pan and add the green chilies, black beans, enchilada sauce, cottage cheese, and Greek yogurt. Stir to combine, then mix in the cut-up corn tortillas until evenly distributed.
Transfer to a baking dish: Lightly grease a 9x13-inch baking dish and pour in the mixture, spreading it out evenly.
Top with cheese and bake: Sprinkle shredded Mexican cheese over the top. Bake at 350°F for 15-20 minutes, or until the cheese is melted and bubbly. Remove from the oven and let the casserole sit for 5 minutes before serving.
