Peel, slice and wash the potatoes.
Put potatoes in a large casserole dish. Cover with water. Add bay leaves, rosemary and salt. Leave to cook on medium heat for 40 minutes or until potatoes are cooked.
Reserve 250 ml. (1 cup or 8.3 oz) of cooking water. Sieve.
Mash potatoes using an electric hand potato masher.
Add egg and butter. Mix.
Season with nutmeg, pepper and salt.