Heat the oven to 180c/fan 160c/gas 4. Peel the Desirée potatoes then slice them about 4mm thick on a mandolin. Heat the double cream with the milk and garlic until it comes to the boil, then drop batches of the potato slices (separating the slices as you go) into the cream mixture and cook them for about 3 minutes, until just cooked.
Remove the potatoes from the cream mixture and reserve the liquid. Put 2 layers of potato in an ovenproof dish, then add enough cream mixture to just cover them. Season. Repeat with more potatoes, cream mixture and seasoning until the dish is full. Sprinkle on the grated emmental and bake for 30 minutes at 180c/fan 160c/gas 4.