Preheat the oven to 350 degrees. Place the tortillas in a covered oven-safe dish and warm them in the oven while you're preparing the other ingredients. This will make them easier to roll.
Chop the sweet potatoes into bite-sized pieces (½ inch cubes). Place 3 cups of water in a large pot, and put the chopped sweet potatoes in a steamer basket in the pot. Cover the pot and bring to a boil, steaming the sweet potatoes until tender (about 10 minutes).
Meanwhile, in a large skillet, saute the onion in the avocado oil for about 5 minutes, or until tender and translucent. Add 1 cup of enchilada sauce, the steamed sweet potatoes, black beans, and diced chilis to the pan (Rinse the beans first if you're using canned). Simmer for a minute.
Mash the filling a bit with a spoon, then fill each tortilla with 2 tablespoons of cheese and 1/3 cup of the sweet potato filling. Roll the tortillas carefully and place them seam side down in a greased 9 by 13 inch pan.
Spread the remaining enchilada sauce on top of the enchiladas, then cover the pan with foil. Bake covered for 20 minutes, or until the cheese is melted. Top with your favorite enchilada toppings (e.g. jalapenos, sour cream, avocado) and serve warm.