Heat olive oil in a large dutch oven or soup pot. Saute onions on medium-low heat until onions are very soft and starting to take on some color, 5-7 minutes, being careful not to burn.
Add garlic and continue to cook for up to a minute.
Add chili powder, oregano, cumin, salt & pepper and saute for another minute or two, until the spices are well blended and fragrant.
Add the grated sweet potatoes and stir well to combine with a wooden spoon. Continue cooking everything over medium heat, covered, until the sweet potatoes are cooked through and tender, 10-15 minutes, stirring every few minutes.
On each tortilla, sprinkle 2 T of cheese down the center diameter of the tortilla, and top the cheese with a heaping 1/4 cup of sweet potato filling. Roll up and place in greased 9 by 13 inch baking pan.
Continue rolling and filling burritos until you have at least 8 burritos, using as many tortillas as your filling requires. I sometimes cram a few more burritos in the pan or you can place any extra burritos into a smaller baking pan.
Sprinkle remaining cheese on top of burritos.
Bake in a 375 degree oven for 20-25 minutes until cheese is bubbly and burritos are hot throughout. Garnish with chopped green onions, and serve with black beans, spicy salsa and guacamole.