Cut the lamb into pieces suitable for serving. Skin the kidney, cut in half lengthways, cut out the white fatty core and cut the kidney into pieces. Peel and slice the onions, carrot, and turnip (if used). Peel and slice the potatoes and cut into thick slices (1/2in/1.25 cm).
Heat the oil in a frying pan, and brown the lamb pieces over a medium heat, turning them so that they cook on all sides. Brown the kidney, and arrange all the meat in a casserole or ovenproof dish. Fry the onion in the pan for 3-4 minutes, to soften it. Add the sliced carrot and turnip (if used) and continue to fry gently, stirring all the time, for a further 3 minutes.
Add the vegetables to the meat in the casserole. Dissolve the stock cube in 2 cups of boiling water. Sprinkle the flour over the remaining juices in the frying pan, and stir. Gradually stir in the stock, stirring hard to make a smooth gravy and adding the salt, pepper and herbs. Pour the gravy over the meat in the casserole, to cover the meat and vegetables.
Then cover the meat with a thick layer of potato slices, placing them so that they overlap and form a thick crust. Dot with the butter. Cover with a lid or piece of tight-fitting foil, and cook in a moderate oven (325F/170C/Gas Mark 3-4) for 1 1/2-2 hours, removing the lid for the last half hour of cooking time, to brown the top. If the top does not seem to be getting crispy enough, either increase the oven heat to 400F/200C/Gas Mark 6-7, or pop the casserole dish under a hot grill for a few minutes.
If you have cook the casserole on top of the stove because an oven is not available, simmer the casserole very gently for 1 1/2-2 hours, then brown the potato topping under the grill as described above.