Poke a few holes in the sweet potato with a fork or knife, then place it on a microwave-safe plate. Microwave the sweet potato for about 8 minutes, or until very tender and easily pierced with a fork (Note 1). Cut it open when done and let it cool for a few minutes while you move on to the next steps. Once cool, remove the inner flesh from the sweet potato with a spoon and lightly mash it with a fork. Discard the peel.
Coat the bottom of a large skillet with the oil and place it over medium heat. Once the oil is hot, add the onion. Sweat the onion for about 4 minutes, until it starts to soften.
Stir in the minced garlic and cumin. Cook the mixture for about 1 minute, stirring constantly, until it becomes very fragrant.
Add the mashed sweet potato, black beans, and adobo sauce to the skillet. Cook the mixture for 1 to 2 minutes, stirring constantly.
Remove the skillet from heat, then stir in the salt and cilantro. Taste-test the mixture and adjust any seasonings to taste.
Preheat the oven to 400°F.
Spread 1 ½ cups of enchilada sauce into the bottom of a 9 x 13 baking dish (Note 2).
Working with three or four tortillas at a time, place them on a dish and cover them with a damp paper towel. Microwave the tortillas for about 20 seconds, until they're warm, moist, and pliable.
Spoon about ¼ to ⅓ cup of the filling into the center of a tortilla, arranging it in a strip. Roll the filling up in the tortilla, then place it, seam side down, into the prepared baking dish. Brush it with some of the remaining enchilada sauce. Repeat this process until all of the filling has been used.
Bake the enchiladas for about 20 minutes, until the sauce is bubbly.
Let the enchiladas sit for at least 5 minutes when they come out of the oven. Then divide them into plates, top with toppings of choice, and serve.