Preheat the oven to 400 degress Farenheit. Prick the potatoes with a knife in several places and bake in a small baking pan until very tender, 1 to 1 1/2 hours.
Meanwhile, heat 2 tablespoons vegetable oil in a large skillet over medium heat. Add the onion and cook, stirring often, unyil softened, about 5 minutes. Add the garlic and for 1 minute longer. Stir in cumin and chili powder and cook for 1 minute. Add the beans and vegetable stock, cover, cook until the beans are very soft and the flavors have blended, about 5 minutes. Use a fork or potato masher to coarsely mash the beans. Season with salt and pepper. You should have about 1 1/2 cups.
Remove the potatoes from the oven and cool. (Keep the oven at the same temperature.) When the potatoes are cool enough to handle, halve and scoop out the flesh. Place the fresh in a small bowl and use a fork to mash coarse. Discard the skins. Add the nutmeg and season with salt and pepper. You should have 1 1/2 to 2 cups.
Spread the tortillas with roughly equal amounts of the bean mixture. Top with equal amounts of the potato puree. Sprinkle on equal amounts of scallions and chesse. Roll the tortillas into cylinder and arrange, seam side down, on a baking sheet. Brush the tops with vegetable oil. Place in a baking pan large enough to hold the burritos in one flat layer. For a soft burrito, cover with foil and bakie until warmed through, about 20 minutes. Bake uncovered for a crisper burrito. Arrange the garnishes attractively in bowls and let diners garnish their own burritos.