As always, there are many different recipes for cooking a leg of lamb. Elise cooks her leg of lamb with the bone in by starting at 425 degrees for 20 minutes then turns the oven down to 300°F and continues to roast the meat for about 10-12 minutes per pound for medium rare lamb.esides, it just fit into my 15-inch long roasting pan, and my larger roasting pan is not as photogenic! The steps involved in making this boneless leg of lamb are: 1  Make a “spice-rub” with garlic, thyme, rosemary, oregano and black pepper. 2  Rub it all over the roast.