Preheat oven to 375 degrees F.
Heat olive oil in a large skillet over medium heat. Add the onion and sweet potato and sauté for 5 minutes, stirring often, until the sweet potato begins to lightly brown. Add 1/2 cup water to the pan, cover, and continue cooking until sweet potatoes are tender, about 5-10 more minutes. Taste a sweet potato to check that it is done. They should be soft but not mushy.
Uncover the pan and stir in the garlic, chili powder, cumin, salt and pepper. Cook for 30 seconds and then remove the pan from the heat.
Stir in the black beans.
Place 1/2 cup of the enchilada sauce in the bottom of a 9x13-inch baking dish.
Assemble the enchiladas: Working with one tortilla at a time, place about 1/2 cup of the sweet potato black bean filling down the center of the tortilla. Add about 2 tablespoons of cheese, roll the tortilla up tightly and place seam-side down in the baking dish.
Pour the remaining enchilada sauce over the enchiladas and top with the remaining cheese.
Cover the pan with foil and bake for 15 minutes. Uncover and bake for 5-10 minutes more, until the enchiladas are hot and bubbly. Serve with toppings as desired.