Put all ingredients for enchilada sauce in a medium saucepan, except for lime, and bring to a simmer. Whisk ingredients and let simmer 10-15 minutes to allow time for sauce to thicken. Add lime juice and turn off heat.
Steam chopped sweet potato in metal steamer basket until tender, about 10-15 minutes.
While sweet potato steams, sauté onion and garlic in a large skillet over medium heat. Make sure to add some water to skillet so nothing burns. No oil is necessary, just keep adding small amounts of water.
Add chili powder, cumin, garlic and black beans. Cook for several minutes, then add in sweet potato. Mash sweet potato while stirring so there aren’t huge chunks. Mix until all ingredients are combined.
Next, turn on oven to 375º.
Use a 9 x 13-inch baking dish and cover bottom with about 1½ cups of enchilada sauce. Dip one side of a tortilla in some of the sauce, fill it with a couple spoonfuls of filling and then roll it up and set it seam-side down in the dish. Keep going until you run out of space or out of filling.
Cover enchiladas with remaining sauce and bake 13-15 minutes. Let cool slightly before serving. Feel free to top with sliced avocado and chopped cilantro. Serve with a side of brown basmati rice.