Preheat the oven to 375°F. Heat the olive oil in a large skillet over medium heat. Add the diced onion and cook for 3 minutes until it softens. Stir in the garlic and cumin, then cook for 1 minute more.

Add the diced sweet potatoes and salt to the skillet. Cook for 8 to 10 minutes, stirring often, until the potatoes begin to soften. Remove the skillet from the heat and stir in the black beans.

Spread a few tablespoons of enchilada sauce in the bottom of a 9x9 baking dish. Layer 4 tortillas over the sauce, tearing them to fit as needed. Top with half the sweet potato mixture, then drizzle with more sauce and sprinkle with half the cheese. Repeat the layers with the remaining tortillas, filling, sauce, and cheese.

Bake for 25 minutes until the cheese melts and the edges bubble. Let the casserole rest for 5 minutes before sprinkling with chopped cilantro.