For the lamb jus, first make some lamb stock. Put the lamb bones into a large saucepan with a little oil and fry over a medium to high heat for 8-10 minutes, until lightly browned all over. Add the onion, celery, carrot, thyme and bay leaf to the pan.
Pour over the water and bring the mixture to the boil. Reduce the heat until the stock is simmering, then continue to simmer for 45 minutes, skimming the foam from the surface of the liquid at regular intervals.
Remove the stock from the heat and strain through a fine sieve into a clean jug or bowl; set aside to cool. Once cooled, spoon off any fat that has risen to the surface of the liquid.
While the stock is simmering, make the stuffing. Heat two tbsp of the oil in a large non-stick frying pan over a low to medium heat. Add the bread cubes, in batches if necessary, and fry for 3-4 minutes, or until golden-brown. Transfer the fried bread cubes to a large mixing bowl.
Add the remaining oil to the frying pan and add the leek, onion and garlic. Fry for 2-3 minutes, stirring regularly, until softened but not browned. Transfer the vegetables to the bowl containing the bread.
Stir the capers, lemon zest, mint and parsley into the bread and vegetables until well combined, then season, to taste, with salt and freshly ground black pepper. Set aside to cool.
For the mint sauce, mix the chopped mint leaves and sugar in a small bowl, then crush using the back of a spoon for 1-2 minutes to extract the flavour from the mint. Stir in the white wine vinegar and a dash of olive oil until well combined. Set aside.
For the lamb, place the lamb onto a chopping bard, skin-side down. Cover the lamb with a large sheet of cling film, then flatten the meat into a large, neat rectangle shape, about 3cm/1½in in thickness, using a rolling pin or meat mallet. Season all over with salt and freshly ground black pepper.
With the longest sides of the lamb rectangle facing you, spoon the stuffing mixture horizontally across the middle of the lamb rectangle (from one short side to the other). Wrap the two long sides of the lamb rectangle around the stuffing to fully enclose it. Truss the lamb securely at 3cm/1½in intervals in several places using cooking string. (Tie the two ends first, then the middle, then fill in the gaps. That way the stuffing doesn't get squeezed out.)
Weigh the trussed lamb joint and make a note of the weight, then place the lamb into a roasting tin. (At this stage, the lamb can be chilled for several hours; remove from the fridge and bring back to room temperature before roasting.)
For the boulangère potatoes, heat the oil in a large, non-stick frying pan over a medium heat. Add the onion and thyme sprigs and fry, stirring occasionally, for 8-10 minutes, until the onion has softened and browned slightly. Add the garlic and continue to fry for 2-3 minutes, then season, to taste, with salt and freshly ground black pepper.
Grease a 20cm x 30cm/8in x 12in roasting tin or ovenproof dish with a little oil.
Arrange a layer of potato slices to cover the base of the dish. Sprinkle over one-third of the fried onions. Continue layering the potato slices and onion mixture, ending with a layer of potatoes.
Pour over the stock until it just reaches the top layer of potatoes. Season again with freshly ground black pepper and garnish with a few sprigs of thyme.
To cook the lamb and potatoes, preheat the oven to 200C/400F/Gas 6. Rub the trussed lamb joint all over with the oil and season well with salt and freshly ground black pepper. Roast the lamb in the centre of the oven, and the potatoes at the bottom of the oven. For the lamb, allow 20 minutes of cooking time per 500g/1lb 2oz of weight. (The full cooking time should be approximately 1hour 20minutes).
When the lamb has cooked to your liking, remove from the oven and set aside on a chopping board. Cover with a piece of aluminium foil and a couple of clean tea towels to keep warm, then set aside to rest. Reserve the roasting tray.
Meanwhile, continue cooking the potatoes at the top of the oven for a further 10-15 minutes, or until browned on top.
To make the lamb jus, spoon as much fat as possible from the top of the juices left in the roasting tray, then place the tray over a medium heat and whisk the flour into the juices. Cook for 10-15 seconds.
Gradually add 300ml/10½fl oz of the reserved lamb stock to the flour mixture, whisking after each addition until the mixture is smooth and well combined.
Stir in the redcurrant jelly and the mint sprig and bring the mixture to a gentle simmer. Continue to simmer for 10 minutes, stirring occasionally, then season, to taste. Stir in any juices released from the resting meat, then strain the jus into a warmed jug.
To serve, trim the ends off the roast lamb parcel, then remove the string. Carve the stuffed lamb shoulder into thick slices. Spoon a portion of boulangère potatoes into the centre of each of six warmed plates. Place a few slices of stuffed lamb alongside each and drizzle over the lamb jus and a little mint sauce. Garnish with a few small whole mint leaves.