1. Fry the lamb in a saucepan until sealed, then add the onion and carrots. Continue frying until the onions start to brown. Add the mushrooms and fry for a further 3-4 mins. Add the tomatoes, crumbled stock cube, worcester sauce and salt and pepper. Stir well and bring to the boil. Cover and simmer until tender, about 30 mins.
2. Meanwhile, put the potatoes into a saucepan of cold water. Bring to the boil and simmer until tender. Drain and mash with a little margarine and milk. Season with salt and pepper. Set aside.
3. When the meat mixture is cooked, thicken the liquid if necessary. Place the mixture in a casserole dish and pile and spread the mashed potato on top. Either brown under a grill or bake in a preated oven until golden brown.