Preheat oven to Gas mark 4-5, 180-190°C, 350-375°F.
Bring your lamb shoulder to room temperature and slash across the top.
Brush with oil and season well with sea salt and freshly ground black pepper.
To prepare the crust, place the remaining ingredients in a small food processor and process until finely ground, or chop everything up as fine as you can.
Press the pistachio and orange crust into the top of the lamb shoulder and place in a medium sized roasting tin.
Roast for 35-40 minutes (for medium), or 60 minutes for well done. Cover with foil during cooking if you find the crust is browning too quickly.
Alternatively, check the internal temperature of the meat for your desired level of doneness: 60°C degrees for rare, 70°C for medium and 80°C for well done.
Remove the lamb from the oven, cover with foil and leave for at least 10 minutes, or 20 if you have the time, before serving.