Preheat oven to 350 degrees F. Pierce sweet potatoes a few times with a fork and place on a sheet of aluminum foil. Bake sweet potatoes until soft and easily pierced with a fork, about 1 hour. Remove from oven and slice in half lengthwise; cool slightly.
Meanwhile, stack tortillas and cover in foil. Place in oven (still at 350) and heat 15 minutes.
Spoon out flesh from cooled sweet potatoes and place in a medium bowl; discard skin. Add lime juice, cumin, cinnamon and nutmeg and mash until smooth and well combined.
Top warm tortillas evenly with sweet potato mixture, red onions, black beans, shredded cheese, sour cream and salsa. Sprinkle with chopped cilantro. Fold in sides and roll up to seal. Serve warm.
To freeze, cover wrapped burritos with aluminum foil and place in freezer. When ready to serve, heat burritos, still wrapped in foil and placed on a baking sheet, in oven at 350 degrees F for 35-40 minutes until warm.