Preheat oven to 180°C. Using a thin sharp knife, prick the lamb all over in 20 places, piercing until you hit the bones. Lay the sliced onions, chillies and bay leaves in the bottom of a deep-sided roasting dish, lay the lamb shoulder on top and pour over 150 ml hot water. Mix the peppercorns, rosemary, cardamom and sea salt together and sprinkle this over the lamb. Lay a sheet of baking parchment on top of the lamb and then seal the dish tightly with foil. Place in the middle of the oven and bake for 2½ hours. Take the foil off the dish along with the baking parchment and ladle the pan juices over the lamb and bake for another 25 minutes. At this point the lamb should be tender and you should be able to pull the meat away from the bone. Cover lightly with foil and rest in a warm place for 10 minutes.
Fifteen minutes before the lamb is ready, place the potatoes in a pot with half the mint and some salt, and cover with warm water. Bring to the boil and cook until ready. Drain, discard the cooked mint, add the leaves from the remaining mint and the oil and toss together, then keep covered in a warm place.
As soon as the lamb begins its 10 minutes’ resting period, bring a pot of lightly salted water to the boil and add the peas, then cook for 2 minutes. Place the ginger and butter in a pan and slowly cook until the ginger begins to sizzle. Drain the peas and add these to the butter, season with plenty of freshly ground black pepper and toss it all together adding salt as needed.