1. Bring lamb to room temperature. Preheat oven to 450°F. Using the point of a sharp knife, make 1-inch deep incisions over surface of lamb. Fill each incision with a piece of sliced garlic and a few rosemary leaves, pushing them completely into the meat. Rub lamb all over with olive oil and lemon juice. Season generously with salt and pepper.
2. Place lamb on a rack set inside a large roasting pan. Roast lamb, uncovered, for 15 minutes. Reduce oven temperature to 350°F. Continue roasting lamb until a meat thermometer inserted into center reaches 135°F for medium, about 18 minutes per pound (about 90 minutes for a 5-lb. roast).
3. Transfer lamb to a carving board and cover loosely with foil; let rest for 15 to 20 minutes. Skim excess fat from roasting pan and place pan over 2 burners on stovetop. Pour in broth and bring to a boil over medium-high heat, scraping up any browned bits on bottom of pan. Cook until liquid has reduced and slightly thickened, about 5 minutes. Taste; season with salt and pepper, if desired. Thinly slice lamb and serve with pan juices.