Let stand at room temperature for 30 minutes. Preheat oven to 425 degrees F. Roast lamb in middle of oven for 20 minutes, reduce heat to 350, and cook until an instant-read thermometer inserted 2 inches into thickest part of meat (not touching bone) registers 135 degrees F, about 1 3/4 hours.et stand at room temperature for 30 minutes. Preheat oven to 425 degrees F. Roast lamb in middle of oven for 20 minutes, reduce heat to 350, and cook until an instant-read thermometer inserted 2 inches into thickest part of meat (not touching bone) registers 135 degrees F, about 1 3/4 hours.