Set your oven to 375°F (190°C) and grease a 9x13 inch baking dish lightly.
Warm a sauté pan on medium heat, then add the diced onions and minced garlic, cooking until soft and clear. Toss in the spinach and cook just until it wilts, then take it off the heat.
In a bowl, combine the sautéed onion, garlic, and spinach with shredded chicken, black beans, diced tomatoes including their juices, sour cream, chicken broth, cumin, chili powder, smoked paprika, salt, and pepper; stir everything together well.
Spread a layer of the chicken mixture on the bottom of your baking dish, place a layer of corn tortillas over it, and sprinkle with shredded cheese. Continue layering in this order until you use all the ingredients, finishing with cheese on top.
Cover the casserole with foil and bake for 25 minutes. Remove the foil afterward and bake for another 10 minutes until the cheese is bubbly and golden.
Allow the casserole to rest for 5 minutes after baking before serving. Garnish with fresh chopped cilantro and slices of avocado if you like.