Preheat the oven to 170°C.
Rub the lamb shoulder with olive oil and season with salt and pepper.
Place rosemary, garlic, onion, and 200ml water in a roasting tin and top with lamb.
Cover the tin with foil and roast for 3 to 4 hours, basting periodically.
Remove foil after 2 hours and continue roasting until the meat tenderly falls off the bone.
Rest the lamb while boiling the potatoes and mint sprigs for 10 to 15 minutes.
Mix the softened butter with chopped wild garlic and seasoning.
Combine chopped mint, sugar, and vinegar in a bowl for the sauce.
Drain potatoes and stir in some wild garlic butter.
Shred the lamb and serve with gravy, potatoes, and mint sauce.