1 Put the potatoes in a saucepan and cover with cold water. Add a pinch of salt, cover and bring to a boil. Cook for about 25 minutes until tender.
2 Drain the potatoes and mash with the butter, milk and season with salt, pepper and nutmeg. Beat in the egg.
3 Blanch the carrots and turnip in boiling salted water for 8 minutes. Drain and set aside.
4 Preheat the oven to 200°C (180° fan) 400°F gas 6. Grease a baking dish.
5 Heat the oil in a frying pan and add the lamb. Stir until browned, then stir in the onion, peas, stock and tomato purée.
6 Tip into the baking dish. Spread the mashed potatoes over the meat and bake for 30-35 minutes until the potatoes are browned and the filling is piping hot.