Preheat the oven to 150C. Get a heavy roasting tray over a high heat.
Prepare your lamb by making several centimetre-deep slashes across the surface. In a food processor combine the nori, rosemary and garlic along with a pinch of sea salt and plenty of black pepper. Whizz up until it makes a dusty breadcrumb, then add enough olive oil to make a slushy paste. Rub the paste generously all over the meat.
Add the lamb to the pan and brown on all sides. This should only take a couple of minutes. Transfer the leg to a roasting bag and put back into the tray. Roast for 1.5 - 2 hours, or until the thickest part of the lamb has come up to 68C (for medium lamb, how I like it) when tested with a meat thermometer.