Prick the sweet potatoes all over and place in the oven at 400 for 20-40 minutes until they are soft when you grab them. The bake time depends on the thickness of your sweet potatoes. If they are very thick, cut them in half and place on a baking sheet to speed up the process. Remove, place in the fridge or freezer and rest until cool enough to handle them.
Wash and finely chop the greens. Place in the skillet over medium heat while they are still damp. Toss lightly until they are wilted down and most of their liquid has steamed off.
Make the enchilada sauce by blending the cilantro or spinach with the yogurt and green onions in a blender until evenly combined. Pour about a third of the sauce into the casserole dish.
Remove the skins from the sweet potatoes and mash in a bowl with the cumin and salt. Gently fold in the drained black beans, cooked greens and half of the shredded cheese.
Warm each tortilla in the skillet over low heat until just barely warm and pliable. Dollop in the filling, roll up and place in the casserole dish. Repeat with the remaining tortilllas until the dish is packed and the filling is used up. Pour on the rest of the enchilada yogurt sauce. Sprinkle the remaining cheese on top.
Place in the oven at 350 for 20-30 minutes until the cheese is melted and the center is hot. It may need a bit more time if you started with cold sweet potatoes.