Over medium-low heat, in a large saucepan with extra virgin olive oil sautè the onion until tender, about 4 minutes. Add the turkey, break up with a fork, and cook it is no longer pink. Drain off any fat. Add the sweet potatoes, cumin, sage, salt and pepper. Stir and cook until potatoes are tender but not falling apart, approximately 5 - 10 minutes.
When the sweet potatoes are tender, sprinkle the mozzarella on top, cover then turn off the heat.