For the roast lamb, preheat the oven to 170C/325F/Gas 3.
Mix the salt, rosemary and thyme together in a bowl and rub all over the lamb shoulder.
Set aside for at least one hour, preferably up to four hours.
Remove the salt from the lamb shoulder, rinse and pat dry. Rub the lamb with the olive oil
Place the onions into a flameproof roasting tin, place the lamb on top and cover with aluminium foil.
Roast in the oven for two hours, basting occasionally, them remove the aluminium foil and roast for a further 1-2 hours, or until the meat is very tender.
Remove the lamb from the tray and set aside to rest.
Place the tray back on the hob, and deglaze with the white wine. Continue to cook until the volume of the wine has reduced by half. Stir in the flour and cook for 1-2 minutes, then add the stock and cook for a further 2-3 minutes.
Strain the gravy through a sieve into a clean saucepan, then season to taste with salt and freshly ground black pepper.
Meanwhile, for the roast potatoes, place 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.
Meanwhile, heat a lidded frying pan until hot, add the butter and spring greens and fry for 2-3 minutes then add the water and cover with a lid. Cook for a further 2-3 minutes, or until the greens have wilted.
Season with salt and freshly ground black pepper.
For the mint sauce, place the chopped mint into a small bowl.
Heat the vinegar and sugar in a saucepan until simmering and pour the mixture onto the mint. Stir until well combined.
To serve, using a knife and fork, pull large pieces of meat away from the bone and arrange on a large serving place. Set some roasted potatoes alongside with the spring greens. Finish with a spoonful of mint sauce and a good drizzle of gravy.