Preheat oven to 425F. Roast tomatillos, onion, sweet potatoes, jalapeño and garlic in olive oil for 30 minutes. Place tomatillos, 1/4 of the onion, garlic, jalapeño, cilantro, lime juice and 1/2 tsp salt in a blender and pulse until you have a chunky sauce.
Preheat oven to 375F. Roughly chop the onion you set aside and place it in a bowl with the roasted sweet potatoes and black beans and mix.
Dip tortillas in the tomatillo sauce and fill with sweet potato mixture. Roll and place in a baking dish. Add remaining sauce over the top. Top with cheese and bake for about 30 minutes.
Serve with sour cream, extra cilantro, avocado or lime if desired.