Peel the potatoes, trim into even blocks and cut into 5 mm. cubes. Place in a container and run cold water over them for 5 minutes. Drain and dry the potatoes on a clean kitchen cloth. Heat the clarified butter in a large cast-iron pan and add the potatoes, garlic and thyme. Season with salt and freshly ground pepper and sauté the potatoes for 8–10 minutes. Frequently toss until tender and golden brown. Add the whole, unsalted butter and continue to sauté for a further 2 minutes. Drain the potatoes and remove the garlic and thyme. Adjust the seasoning if necessary. The potatoes can be sautéed in advance and reheated in a little clarified butter before serving.