To make the marinade, add the oil, Chimichurri, garlic, lemon zest and juice, and salt to a large bowl. Mix to combine.
To prepare the lamb, poke 10 small incisions into the meat with the point of a sharp knife, then rub the marinade all over it. Marinate for at least 30 minutes or preferably overnight.
Preheat the oven to 170°. Let the lamb come to room temperature for about 30 minutes.
Prepare the potatoes by peeling them and scoring with a sharp knife in the shape of an x, careful not to cut the whole way through.
Make the potato seasoning by combining the oil, Chimichurri, Paprika and salt in a bowl. Pour over the potatoes and set aside.
Prepare your roasting tray. Firstly, line the tray with 2 long pieces of foil, the first one going length wise and the next one going width wise, forming a cross shape. Each piece should overhang past the tray on each side by about 20cm. *If your roasting tray is very long, you may need two pieces of foil side by side to cover the base of the tray.
Repeat the same process with the baking paper layering in a cross pattern on top of the foil.
Scatter the potatoes on top of the baking paper along with the halved garlic bulbs.
Place the lamb and marinade on top and distribute the water around the potatoes.
Take the overhanging layers of baking paper and gently tuck in the lamb, wrapping it as close as possible. Next wrap the baking paper parcel with the layers of overhanging foil to form an airtight seal. Place the lamb in the oven and cook for 3 ½ hours.
Remove the tray from the oven and carefully open the parcel. Remove the potatoes and return the lamb, uncovered, to the oven for a further 30 minutes to crisp up.
Serve on a platter surrounded by the roast potatoes and drizzle juices from the pan over the top.
