These potatoes go well with lamb.
Boil and drain the potatoes and pass through the mouli-légume.
Gently cook the leeks in the butter until softened; be careful not to allow them to colour. Purée the leeks in a blender. Mix together the potatoes, leek, butter, milk, salt, pepper and nutmeg. Put the mixture in a buttered gratin dish and sprinkle grated Caerphilly cheese on top. Bake the potato and leek gratin in an oven set at 200°C/400°F/gas 6 until the cheese is bubbling and golden.