Set the oven at 180C/gas mark 4. In a very large pot, sizzle the lamb in a little oil over a moderate to high heat, until the surface is golden. Turn over and brown the other side then remove from the pan.
Peel the shallots, cut into 1 cm thick slices, then let them cook for 10 minutes in the lamb pot, adding a little more oil if necessary, until they are light gold and translucent.
Return the lamb to the pan, add the rosemary and thyme, a seasoning of black pepper and the stock, then bring to the boil. Cover the pan with a lid and place in the oven, leaving it to bake for 90 minutes, until the lamb is tender.
Scrub the potatoes and cut into ½cm slices. Remove the pot from the oven then take out the lamb and place it on a chopping board, preserving the stock.
Tweak the bones from the meat – they should slide out effortlessly – then cut the meat and fat into pieces. Mix the meat with the shallots from the cooking liquor. Turn the oven heat up to 200C/gas mark 6.
Place a layer of potatoes on the bottom of a large baking dish, then spread a layer of the lamb and shallots over them, followed by a layer of potatoes. Continue layering potatoes and meat, seasoning with salt and pepper as you go, finishing with a layer of potatoes.
Pour the stock over the potatoes, letting it run down through the layers. Bake for an hour to 90 minutes, until the potatoes are golden and totally tender.