Preheat and prep: Heat your oven to 375°F (190°C). Lightly mist a 9x13-inch baking dish with cooking spray.
Sauté aromatics: Warm oil in a large skillet over medium heat.
Add onion and bell pepper. Cook 4 to 5 minutes until softened. Stir in garlic for 30 seconds.
Build the filling: Add beans, corn, drained tomatoes, salsa, taco seasoning, and chicken broth.
Stir well and simmer 3 to 4 minutes to marry flavors.
Add chicken: Fold in the shredded chicken and 2 tablespoons cilantro if using. Taste and adjust salt, pepper, or heat.
Layer the casserole: Cut tortillas into halves or quarters. Arrange a layer to cover the bottom of the baking dish.
Spoon a third of the chicken mixture over. Sprinkle a little cheese. Repeat for 2 to 3 layers, ending with cheese on top.
Bake: Cover with foil and bake 15 minutes.
Uncover and bake another 8 to 10 minutes, until cheese is melted and edges are bubbly.
Rest and finish: Let the casserole rest 5 to 10 minutes. Top with remaining cilantro and a squeeze of lime. Add optional toppings right before serving.