Preheat the oven to 400 degrees F. Toss the sweet potato cubes with olive oil, cumin, salt, and pepper on a baking sheet. Roast for 20 minutes until the edges start to brown.
While the sweet potatoes roast, heat a skillet over medium heat. Add the diced onion and cook for 5 minutes until softened. Stir in the garlic and cook for 1 minute more.
Remove the skillet from heat. Add the roasted sweet potatoes and drained black beans to the skillet. Stir gently to combine.
Spread a few tablespoons of enchilada sauce across the bottom of a 9x9 inch baking dish. Layer 4 tortillas over the sauce, tearing them to fit as needed.
Spoon half the sweet potato and bean mixture over the tortillas. Pour half the remaining enchilada sauce over the top and sprinkle with 1 cup of cheese. Repeat with the remaining tortillas, filling, sauce, and cheese.
Cover the dish with foil and bake for 20 minutes. Remove the foil and bake for 10 minutes more until the cheese is bubbly and lightly browned.