Pre-heat the oven to gas mark 4, 180c.
Rub a gratin dish all over with the garlic and butter it liberally. Put one
layer of potatoes, a little salt, pepper, Gruyere, onion, and parsley in the
dish. Then add a layer of ceps, onion, parsley, and Gruyere. Continue the
layers, finishing with a layer of potatoes and cheese. Dot with butter and
pour over the double cream. Bake in the oven for 1 3/4 hours. (From Savoy.)