1) Preheat oil in large cast iron skillet or deep-fryer.
2) Place sweet potatoes in a large saucepan and cover with cold water. Bring to the boil, reduce heat, and cook until slightly tender, about 10 minutes. Drain the sweet potatoes well in a colander.
3) Add sweet potatoes to skillet or deep-fryer. Cook undisturbed for 5 to 6 minutes until nice and crisp. Garnish with sugar or salt, to taste.