Preheat the oven to 375°F (190°C).
In a large skillet over medium heat, sauté the diced onion for about 3-4 minutes until soft.
Add the shredded chicken, black beans, cumin, chili powder, salt, and pepper to the skillet. Stir well to combine and cook for another 2-3 minutes until heated through.
In a mixing bowl, combine 1 cup of enchilada sauce with the chicken mixture, mixing until evenly coated.
Warm the corn tortillas in a microwave for about 30 seconds to make them pliable.
In the baking dish, spread a thin layer of enchilada sauce on the bottom.
Take a tortilla, spoon about ¼ cup of the chicken filling into the center, then roll it up tightly and place it seam-side down in the baking dish. Repeat for all tortillas.
Pour the remaining enchilada sauce over the rolled tortillas, then sprinkle with the shredded cheese.
Bake in the preheated oven for 20 minutes, or until the cheese is melted and bubbly.
Let cool for a few minutes before serving. Garnish with fresh cilantro and serve with sour cream if desired.