Mix together the garlic, oregano, olive oil and lemon juice in a pestle and mortar with some sea salt and black pepper.
Rub the mixture into the lamb shoulder, pushing it inside the meat. If it's tightly rolled give it a stab a few times with a knife and rub the mixture in.
Let the lamb sit for at least an hour to soak up the marinade.
Peal and chop the potatoes and add them to the bowl of the pressure cooker, add a little water or stock and then place the lamb on top.
Cook on high pressure for 45 minutes and then allow the pressure to release naturally for 15 minutes.
Close the lid and air fry the the contents for 20 minutes, turn the lamb over half way through if required.
Serve the lamb and potatoes in a bowl with some crusty bread to soak up the sauce.