1. To make Sauce: Bring all ingredients to a simmer in saucepan over medium heat. Whisk to combine, then remove from heat. Season with salt and pepper, if desired.
2. To make Filling: Heat oil in separate saucepan over medium heat. Add onion, and sauté 3 to 5 minutes, or until soft. Add sweet potatoes, tomatoes, salsa, garlic, chipotle chile, and 1/2 cup water; bring to a boil. Reduce heat to medium-low, and simmer 30 to 40 minutes, or until sweet potatoes are soft. Mash mixture with potato masher until combined. Add black beans, and cook 5 minutes. Stir in half of queso fresco, and remove from heat.
3. To assemble Enchiladas: Preheat oven to 350ºF. Brush 13- x 9-inch baking dish with oil. Spread 1/2 cup Sauce in bottom of dish. Fill tortillas with Filling. Roll, and pack close together seam-side down in baking dish. Top with remaining Sauce and queso fresco. Bake 15 minutes. 4. Adjust oven to broil. Broil Enchiladas 5 minutes, or until cheese is browned and bubbly. Let rest 10 minutes before serving. Garnish with lime wedges, avocado slices, sour cream, and cilantro sprigs, if using.