For the lamb, place the onion, carrot, celery, garlic, thyme and rosemary in the bottom of a large, deep roasting tin. Put the lamb on top and season with salt and pepper. Drizzle with the olive oil and pour the stock in the bottom.
Roast for 30 minutes uncovered. Then lower the temperature to 160C/140C Fan/Gas 3. Cover the roasting tin with aluminium foil and cook for a further 4 hours.
For the mint sauce, put the chopped mint in a small bowl and add the vinegar and sugar and stir until well combined. Set aside until ready to serve.
For the roast potatoes, put the potatoes into a large saucepan, cover with water, add a pinch of salt and bring to the boil. Reduce the heat and simmer for 4-5 minutes.
Meanwhile, place the beef dripping into a large roasting tray and preheat in the oven.
Drain the potatoes in a colander and shake around a little to roughen up the edges.
Add the potatoes to the preheated fat and turn carefully, or until coated in the fat. Sprinkle with salt and place in the oven for 30 minutes, until starting to turn golden-brown and crisp. Turn the potatoes and return to the oven for a further 20-30 minutes.
When the lamb is very tender, remove it from the cooking juices and allow to rest. Keep warm until ready to serve.
For the glazed parsnips and cabbage, preheat the oven to 200C/180C Fan/Gas 6.
Place the parsnips, sherry and honey in a baking tray and mix to coat the parsnips. Season with salt and pepper. Roast for 20 minutes, turning half way through, until the parsnips are tender and golden-brown.
Heat a little oil in a large frying pan and stir fry the cabbage for 1-2 minutes, then add the butter and stir to coat.
To serve, shred the lamb and place on serving plates. Place the roast potatoes, parsnips and cabbage next to the lamb. Serve with the mint sauce, gravy and grated chestnuts (if using).