To prepare the shoulder of lamb, preheat the oven to 200C/Gas Mark 6. Open out the lamb shoulder and cover the meat with the cumin, fennel seeds, half the rosemary, salt and pepper. Roll into a large sausage and tie with string. Heat the oil in a heavy-bottomed frying pan, add the lamb and brown until golden all over. Set aside.
Sweat the mirepoix until softened then place in an ovenproof dish. Add the garlic cloves and sprinkle with the rest of the rosemary and butter. Put the lamb in the dish and cover with stock before roasting for 3-4 hours. Remove and leave everything for 15 minutes.
To cook the rack of lamb:
Heat a large heavy-bottomed frying pan with a tablespoon of olive oil. Season the lamb with salt and pepper, and brown them in the pan on a high heat for about 2-3 minutes on each side until golden. Transfer to a roasting dish and roast in the oven for another 6-8 minutes. Take the lamb out of the oven and leave to rest for 5 minutes on a cooling rack.
To prepare the potatoes:
Cut the potatoes in half and place in a non-stick frying pan. Add enough water to cover the potatoes. Add the butter, crushed clove of garlic, salt and rosemary. Place a lid of tin foil over the pan and put on a high heat.
Cook until all the liquid reduces – don’t be scared here and don’t shake the pan or you won’t get the right results. As the water disappears, the butter will start to caramelise. The secret is to let the butter caramelise around the potatoes until they are golden all over. Remove from heat and allow to cool.
Remove the string from the shoulder of lamb and carve the meat into slices, then carve the rack of lamb. Assemble 4 or 5 potatoes on each plate, then place the lamb over the top, adding the spring onions. Pour the remaining cooking juices over the top.