In a large bowl combine the oil, salt, sage, cinnamon, and maple syrup and mix thoroughly. Add in the sweet potatoes and toss to coat all slices evenly.
Arrange the potatoes in a cast iron skillet with the large sides of the slices touching. Cover with the foil and cook for 30 minutes. Remove the foil and continue to back for another 15 minutes or until the potatoes are firm but soft enough for a fork to pass through easily.