Scrub the potatoes and slice very thinly, dropping the slices into a bowl of cold water as you proceed. Melt
Cut another tablespoon of the butter into little pats and dab over the potatoes. Sprinkle with half the rosemary leaves and season. Repeat twice using ⅓ each time of the remaining potatoes and
Cook over a gentle heat for about a half hour or until the bottom is crisp and golden brown. Invert the galette onto a flat plate and put it back into the pan. Continue cooking until the other side is golden. Cut into wedges and serve.