Using your hands, rub the lamb all over with lemon juice. Pat the garlic and rosemary evenly all over the surface of the meat. Season the meat with salt and pepper and place the lamb in a roasting pan.
Roast the lamb for 20 minutes. Reduce the oven temperature to 325°F. and continue to roast (general rule is 30 minutes per pound) 2 to 2½ hours, or until meat thermometer inserted into center of the roast registers 145 to 150°F.
Remove the lamb from the oven and allow to rest for about 15 minutes before carving (the temperature will continue to rise about 5 or 6 more degrees).
Notes
The weight of a leg of lamb can vary from 4 to 6 pounds (typically). Cooking time will vary depending on size, as will the number of servings.