For the mint jelly, oil a small roasting tray and line it with cling film.
Add 300ml/10fl oz water and the sugar to a large pan. Bring to a rolling boil for 10 minutes then reduce the heat. Squeeze out any excess water from the gelatine, add the gelatine to the pan and simmer until dissolved. Stir in the vinegar and mint. Remove from heat and set aside to steep for 20 minutes.
Strain the jelly and transfer to the lined tray. Place in the fridge to set overnight.
For the shoulder of lamb, preheat the oven to 200C/400F/Gas 6.
Heat the olive oil in a large ovenproof casserole over a medium heat. Season the lamb shoulder with salt and freshly ground black pepper.
Add the lamb to the casserole and cook, turning regularly, until browned all over. Remove the lamb and set aside.
Add the onions, carrots, leek, celery and garlic and three sprigs of rosemary. Stir well, then return the browned lamb to the casserole and pour over the red wine and stock.
Spread the butter over the parts of the lamb not covered by liquid, then place the remaining rosemary sprig on top. Transfer the casserole to the oven and cook, covered, for two hours.
Reduce the oven temperature to 150C/300F/Gas 2. Remove the lid and cook until the lamb is tender and the top is crisp, this will take another 1-2 hours.
Remove the lamb from the cooking juices and allow to rest. Shred the meat and set aside in a warm place until ready to serve.
Strain the cooking liquor and return it to the casserole. Cook over a medium to high heat until the volume of liquid has reduced by half.
For the beans and peas, place a pan of boiling water over a medium heat. Add the beans and peas and cook for 2-3 minutes. Drain and set aside.
Heat a frying pan and add the butter and 100ml/3½oz water. Once the butter is melted, add the beans and peas and cook for a further minute.
To serve, put the shredded lamb on serving plates and spoon over the reduced cooking liquor. Put the beans and peas alongside. Cut out squares of the mint jelly and serve them around the plate.