Coquilles St Jacques Nantaise

For the potato purée, cut the potatoes in equal pieces and put them in a pot of cold salted water. Cover, bring to a boil and simmer for 15-20 minutes or until very tender. Drain the potatoes thoroughly and work them back into the pan through a sieve or purée in a food mill. Add the butter with the egg yolk, salt, pepper and nutmeg to taste, and beat with a wooden spoon over a low fire until th