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Heat oven to 190C/170C fan/gas 5. Cut small pockets into the skin and flesh of the lamb by piercing it with the point of a sharp knife. Stuff each pocket with a slice of garlic and a few leaves of rosemary, or scatter with thyme. Put in a roasting tin, season well with salt and freshly ground black pepper, then cover with foil.
Peel and thinly slice the potatoes, rinse under the cold tap and pile into a large ovenproof dish or roasting tin. Toss with the onions, remaining slices of garlic and a good scattering of herbs.
Heat the stock and butter together, then pour over the potatoes. Cover the dish with foil and bake in the oven with the lamb for 1 hr. Uncover, put the lamb on top of the potatoes and roast uncovered for 45 mins more. Allow the lamb to rest before carving, for about 15 mins. Leave the potatoes in the oven (covered, if starting to brown too much) until ready to serve.