Preheat the oven to 400°F. Toss the sweet potato cubes with olive oil, cumin, chili powder, and salt on a baking sheet. Roast for 20 minutes until tender.
While the sweet potatoes roast, heat a skillet over medium heat. Cook the onion for 3 minutes, then add the garlic and cook 1 minute more. Stir in the black beans and 1/4 cup of the enchilada sauce. Remove from heat.
Spread 1/4 cup enchilada sauce in the bottom of an 8x8-inch baking dish. Layer half the tortilla pieces over the sauce. Top with half the roasted sweet potatoes, half the bean mixture, and 1/2 cup cheese. Repeat the layers once more.
Pour the remaining enchilada sauce over the top layer and sprinkle with the remaining 1/2 cup cheese. Cover the dish with foil and bake for 20 minutes.
Remove the foil and bake 10 minutes longer until the cheese is lightly browned. Let the casserole rest 5 minutes before serving. Sprinkle with chopped cilantro.