Lightly grease the bottom and sides of a 9x13-inch baking dish with cooking spray, or use a pastry brush to give the dish a light layer of vegetable oil.

Sprinkle the taco seasoning over the shredded chicken in a medium mixing bowl. Add the chicken stock to the bowl and use a pair of tongs to toss the chicken in the seasonings. The chicken should be evenly moistened and coated with taco seasoning. Add the drained black beans to the chicken and use the tongs to toss them with the chicken.

Pour 2 ½ cups of the enchilada sauce into a pie dish or wide, shallow 1 ½-quart mixing bowl. Dip 8 corn tortillas into the enchilada sauce in the bowl, one at a time, ensuring each is evenly coated in the sauce. Line the bottom of the prepared baking dish with the dipped tortillas, overlapping them slightly to completely cover the bottom of the baking dish. Sprinkle ¾ cup of the shredded cheese over this layer of tortillas. Spoon half of the shredded chicken mixture over the cheese in an even layer. Top the shredded chicken with another ¾ cup of the shredded cheese. Repeat the dipping and layering process once more. Top with 8 more sauce-coated tortillas. Pour the remaining enchilada sauce over the top of the assembled casserole. Sprinkle the last of the cheese over the surface and cover the baking dish with aluminum foil.

Bake the casserole for 25 minutes, covered. After 25 minutes, carefully remove the foil and bake until the cheese is completely melted and starting to brown, another 10 to 12 minutes.

Allow the casserole to cool for 10 to 15 minutes. Use a spatula to cut the casserole into 8 squares. Serve the chicken enchilada casserole garnished with sour cream, chopped cilantro, diced red onions, and jalapeño slices. Refrigerate cooled leftovers in a covered container for up to 3 days.