1. Preheat oven to 400°F. Line a baking sheet with aluminum foil (preferably the easy release kind).
2. Peel the sweet potatoes. Cut into strips that are about 1/4 inch wide on each side.
3. Place the sweet potatoes into a large bowl. Add oil, salt, and rosemary. Stir well to thoroughly coat the fries. Stir in 2/3 of the Parmesan. Spread out onto the baking sheet in a single layer. Sprinkle with remaining Parmesan.
4. Cook for 20-25 minutes, turning about halfway through. Fries are done when they are browned around the edges. Transfer immediately to a paper towel lined plate and serve warm.