Preheat the oven to 180 degrees C. Boil or steam the potatoes until tender, then drain and mash well. Heat the cream or milk and half the butter in a saucepan and beat into the mashed potatoes. Season well with the salt, pepper and nutmeg. Cover and keep warm.
Heat the oil in a saucepan, add the onion and garlic and cook over medium heat for 5 minutes, or until soft and golden. Stir in the Worcestershire sauce, tomato sauce and herbs. Stir in the meat and gravy or stock. Transfer to an ovenproof dish and spoon the potatoes carefully over the surface. Smooth the potatoes, then fork the surface decoratively.
Dot the remaining butter over the top of the potato and bake the shepherd's pie in the centre of the oven for 45 minutes, or until golden brown and thoroughly heated through.