Photo: Brett Stevens
First make the Cumberland glaze. Pare the rind off the orange and lemon with a potato peeler, leaving behind the white pith. Stack the strips and slice diagonally into thin slivers about 2cm long. In a medium pan of boiling water, cook the zest and shallot for 2 minutes. Drain.
Halve and juice the orange and lemon, and pour the juice into the saucepan, adding the remaining ingredients and the blanched zest and shallot. Bring to the boil, stirring well, then bubble, uncovered, for 15-20 minutes until syrupy. Transfer to a bowl, cover and leave to cool. The sauce will thicken as it cools but will still be a pourable consistency.
Preheat the oven to 220°C, fan 200°C, gas 7. Calculate the cooking time of your joint at 33 minutes per kg, plus 15 minutes – so about 1 hour for a joint this size. Make cuts all over the top of the lamb using a sharp knife and push a sliver of garlic into each cut. Place the onion in the centre of a roasting tin that will hold the lamb snugly and drizzle over a little oil. Season the lamb all over and put it, fat-side up, on top of the onion, so the onion is tucked underneath and hidden from view.
Pour 1cm of water into the pan. Drizzle a little oil over the lamb and roast for the appropriate time. Baste every 30 minutes, adding more water as necessary; at no time should the pan dry out. This is to stop the onion burning and to provide the basis of the gravy. By the end you should have plenty of golden juices to call on.
Coat the lamb with 3 tablespoons of the Cumberland glaze 15-20 minutes before it is ready; return it to the oven.
When cooked, transfer the lamb to a carving board, loosely cover with kitchen foil and leave to rest in a warm place for 20-25 minutes. Spoon off any excess fat from the roasting tray. If necessary, simmer first to reduce the liquid – in this case leave to stand for a few minutes to allow the fat to separate out after reducing before skimming. Add the sherry and simmer until sticky and reduced. Stir in the flour and gradually work in the stock. Bring to the boil and bubble, uncovered, until the gravy looks glossy and thick, seasoning to taste. Pass it through a sieve into a bowl or jug. Carve the lamb across the grain, adding any juices to the gravy. Serve with the gravy and the rest of the Cumberland glaze.
Get ahead: the Cumberland glaze can be made up to 2 days ahead.
Kitchen secret: an easy-carve or 'carvery' joint of lamb has been deboned then tied back on to the bone, so you get the benefits of roasting on the bone, but it can be simply removed after cooking for easy carving.