Cut the sweet potatoes into cubes about 1 inch in size. Mince the fresh sage and place 2 tablespoons into a bowl with the potatoes. Then add the olive oil and salt and pepper. Mix well so that all the potatoes are coated with the seasoning evenly. Spread out potatoes on a sheet pan and place in a preheated oven at 400 degrees. Roast the potatoes for 25 to 30 minutes or until tender.