When roasting a joint such as a leg of lamb, place the meat directly on the oven rack in a preheated 425 degree oven. Place a wide roasting pan on the rack beneath the joint to catch all the fat and drippings.
Peel the potatoes and place them, whole, in a large saucepan. Cover with cold water and bring to a boil. Simmer for about 5 minutes, then drain. Shake the potatoes roughly in the colander to bruise their sides.
After the roast has been in the oven at least 20 minutes so that fat and drippings have started to collect in the pan, dump the parboiled potatoes into the drippings. Strip the leaves off one or two sprigs of fresh rosemary and toss them over the potatoes along with a sprinkle of salt and pepper. Shake the pan well to coat the potatoes in fat and drippings. Continue to cook, occasionally turning the lamb and shaking the potatoes, for at least another hour and up to about 90 minutes, until the roast is cooked and the potatoes are crisply golden-brown outside and tender inside.