Preheat the oven to 350 degrees F. Grease two 9x13 inch casserole dishes (if you are planning to have leftovers, it's best to wait to assemble into enchiladas before baking so only do one pan).
Heat 3 teaspoons oil in a large skillet over medium heat. Add the onion and cook for 5 minutes or until tender. Add the potatoes and water and bring to a boil, cover and cook for 5-8 minutes or until tender. Push the potatoes to the sides of the pan, add the remaining oil and the garlic and cook for 30 seconds. Mix in the spices, black beans, corn, and 2 tablespoons of the enchilada sauce.
To assemble, place about 1 tablespoon cheese down the middle of a tortilla and then add 1/3 cup of the sweet potato filling. Fold in the sides and place seam side down in the pan, Repeat with remaining ingredients.
Pour the enchilada sauce over the top of the tortillas and then top with cheese. Bake for 15 minutes or until cheese is melted.
Top with tomatoes, avocados, and cilantro. Serve immediately.