Peel the potatoes and using a mandolin or sharp knife, cut the potatoes into thin slices, placing them in the cream as you go, being sure to give them a good coating in the cream to avoid oxidisation. Arrange potato slices in the tin, overlapping and pressing down, trying to build the layers as evenly as possible. Every second layer, pour in some of the cream, season with salt and pepper and sprinkle over some of the rosemary leaves. Make sure that the top layer is only potato.