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Boil the potatoes in salted water for 20 mins or until tender, then drain.
Heat the grill to high. Mash the potatoes until smooth, then beat in the crème fraîche and half the cheese. Season to taste, then spoon the hot mash into a medium baking dish and smooth the top.
Mix together the rest of the cheese, the mustard and thyme leaves. Spread over the mash with a spatula or the back of a spoon. Grill until the cheese is melting and golden, then serve with the beef.