Slice the potatoes
Clean and dry your sweet potatoes, then slice them as thinly as possible. If you have a mandolin, use it.
Preheat the oven to 120 °C / 250 °F. Place a cast-iron pan (or any other heavy-bottom cooking pan) over high heat. Pour in the vegetable oil and heat until the oil reaches 175 °C / 350 °F. Add the thinly sliced sweet potatoes (use an aluminum basket for frying if you have it). Fry for about 2 - 3 minutes. Remove the chips from the oil and transfer them on a paper towel-lined baking sheet.TipIt's important to regularly shake the pan the first minute to assure the crispiness.
Replace the kitchen towels with the new ones and place the baking sheet into the oven. Bake for 8 - 10 minutes at 120 °C / 250 °F.
Using a pestle and mortar smash the sprigs of rosemary and sea salt. Sprinkle the rosemary salt all over the sweet potato chips and enjoy.