Heat the oven to 220C/200C fan/gas 7 or the air fryer to 200C. Put the sweet potatoes in a pan, cover with cold water and sprinkle in a good amount of salt. Bring to the boil and cook for 2 mins. Drain and leave to steam-dry for a few minutes, then toss with the garlic and onion granules, the paprika and oil, and season well. Arrange on a baking tray or in the air fryer basket in an even layer and cook for 25 mins (or 20 mins in the air fryer) until golden and crispy.