Preheat oven to 375. Bake or microwave the sweet potato until tender, peel and slice into cubes. In a large pan over medium heat, sauté the garlic and leeks in olive oil until leeks are tender. Add the black beans, cumin, marjoram, salt and pepper and stir until well combined. Spread a thin layer of tomatilla salsa into the bottom of your baking dish. Lay tortillas on a plate and layer one end with the sweet potato cubes, leek and black bean mixture, and a sprinkle of cheese. Roll up tortilla and place seam side down in row into glass baking dish. Cover tortillas with more tomatillo sauce, a light sprinkle of cheese, and any leftover beans. Bake ~20 minutes, or until cheese is melted and enchiladas start to brown.