Preheat the oven to 375°F and lightly grease a 9x13 inch baking dish.

In a skillet over medium heat, sauté the chopped onion for about 2 minutes until it starts to soften. Then add the minced garlic and continue to sauté for another minute until the onion is translucent and the garlic is fragrant.

Stir in the ground cumin, chili powder, salt, and pepper. Add the shredded chicken and green chilies if using, mix well, and remove from heat.

Warm the tortillas slightly in a skillet or microwave to make them pliable. Spread a thin layer (about ⅓ cup) of enchilada sauce on the bottom of the baking dish.

Place 4 tortillas over the sauce, overlapping as necessary. Top with half of the sautéed chicken and onion mixture, half of the black beans, and a third of the cheese. Drizzle with some of the enchilada sauce (about ⅓ cup).

Repeat the layering with another 4 tortillas, the remaining chicken, onion, black beans, another third of the cheese, and more enchilada sauce (about ⅓ cup).

Top with the remaining 4 tortillas, the rest of the enchilada sauce, and the remaining cheese.

Cover with aluminum foil and bake for 25 minutes. Remove the foil and continue baking for an additional 5-10 minutes, or until the cheese is melted and bubbly.

Let the casserole cool slightly before garnishing with fresh cilantro or green onions. Serve with sour cream and sliced jalapeños on the side, if desired.