Heat oven to 180C/160C fan/gas 4. Peel the potatoes and cut them into cubes. Put the garlic, thyme and oil in a large roasting tin, and put it in the oven for 5 mins.
Tip the potatoes into the roasting tin, quickly turn them to coat in the oil, then spread them out in a single layer. Season well and roast for 1–11/2 hrs, turning occasionally during cooking, until golden and crispy.