Lamb shoulder roasts make a great oven roast.
Preheat your oven to full whack, (450 to 500 degrees F).
Slash the fat side of the lamb all over with a sharp knife. Lay half the sprigs of rosemary and half the garlic cloves on the bottom of a high-sided roasting pan, rub the lamb all over with olive oil and season with salt and pepper. Place it in the pan on top of the rosemary and garlic, and put the rest of the rosemary and garlic on top of the lamb. Tightly cover the tray with aluminum foil and place in the oven. Turn the oven down immediately to 325 degrees F and cook for 4 hours - it's done if you can pull the meat apart easily with 2 forks.