Preheat oven to 425°F.
In a large skillet over medium heat, saute the onions in the olive oil until they begin to soften, about 4 minutes.
Add the cooked chicken breast, black beans, corn, diced tomatoes, and taco sauce. Stir to combine.
Add the chili powder, cumin, and salt and pepper to taste, and stir to combine.
Cook the chicken mixture over medium heat until heated through. Taste and adjust seasonings if needed.
Spray a 9x13-inch baking dish with cooking spray. Place two tortillas in the dish. They will overlap.
Spoon half of the chicken mixture over the tortillas and sprinkle with 1 cup of the shredded cheese.
Top with 2 more tortillas, then spoon the remaining chicken mixture over the tortillas.
Top with 2 more tortillas and sprinkle with the remaining cheese.
Bake, uncovered, for 15 to 20 minutes or until bubbly and the cheese is melted and lightly browned.
Remove from oven and allow to rest for 15 minutes before slicing and serving.
If desired, drizzle each serving with additional taco sauce, spoon on a dollop of sour cream, and sprinkle with chopped green onions and fresh cilantro.