Roast the sweet potatoes and mash using a blender or by hand. Transfer to a large mixing bowl. Add black beans, salsa, chili powder, cumin, and hatch chiles to the bowl and mix well. Transfer to a shallow 2-quart dish and spread evenly.
Spread a layer of Greek yogurt.
Top with romaine lettuce, tomatoes, avocado, scallions, and finally, cheese.
Refrigerate before serving.