Heat the butter over a medium heat in a saucepan. When the butter is foaming, add the potatoes and fry until deep golden-brown on one side, about 5-6 minutes. (Do not move the potatoes as they cook.)
Turn over the potatoes and cook for a further 5-6 minutes, or until golden-brown on both sides.
Carefully pour in the stock, then add the garlic cloves and thyme sprigs. (CAUTION: the hot fat will splutter when it comes into contact with the stock.) Season, to taste, with salt and freshly ground black pepper.
Cover the pan with a lid and reduce the heat until the stock is simmering. Simmer the potatoes until tender, then remove the potatoes from the pan using a slotted spoon and keep warm.