Bone the lamb and put the meat and the bones in a large saucepan, cover with water and season with salt and freshly ground pepper. Place the pan over medium heat and boil gently for 30 minutes.

Drain off the liquid and reserve, allowing it to cool so any fat can be skimmed off the top. While the stock is cooling, cut the meat into fairly large pieces.

Heat the oven to 130°C (250°F) gas 1/2. Put a thickish layer of potatoes in a casserole, sprinkle with parsley and thyme, add a layer of meat, then onions, seasoning each layer well with salt and freshly ground pepper. Repeat until all the ingredients are used, ending with a layer of potatoes.

Pour over the reserved liquid, cover the casserole with greased foil and a lid and place in the oven for 1 1/2 hours or until the potatoes are tender, adding a very little more liquid if the ingredients seem to be drying up. Serve immediately.