In a large mixing bowl, coat chicken breasts with fajita seasoning until fully covered. Heat olive oil in a large skillet over medium heat. Sauté chicken for 5-7 minutes until golden and cooked through, then remove from skillet.

In the same skillet, add sliced onion and bell peppers, cooking for 8-10 minutes until tender, soft, and aromatic. Return chicken to skillet, mix well, and remove from heat.

In a mixing bowl, blend softened cream cheese, condensed cream of chicken soup, and sour cream until smooth and creamy. Add undrained diced tomatoes and green chilies, and fold in shredded cheddar and Monterey Jack cheese.

Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish. Layer half of the cream cheese mixture on the bottom, then place 6 corn tortillas over it, followed by half of the chicken and vegetable mixture and another layer of cream cheese mixture. Repeat layers, finishing with remaining tortillas and cheese mixture.

Cover the casserole with aluminum foil and bake for 25 minutes, then remove foil and continue baking for 10-15 minutes until cheese is bubbly and golden brown. Let rest for 10-15 minutes before serving.

Cut the casserole into squares and serve hot with optional toppings like salsa, guacamole, extra sour cream, or cilantro.