Trim fat from lamb. Make 1/4-inch-deep slits in lamb; insert a garlic slice into each slit. Combine chopped rosemary, salt, and pepper. Sprinkle half of rosemary mixture over lamb. Insert meat thermometer into thickest part of lamb, if desired. Place lamb in a large roasting pan coated with cooking spray; add potatoes, carrots, and beef broth.
Sprinkle vegetables with remaining rosemary mixture. Bake, uncovered, at 350° for 2 hours or until meat thermometer registers 150° (medium-rare) to 160° (medium), stirring vegetables occasionally.
Transfer lamb to a serving platter; arrange vegetables around lamb, and keep warm. Skim fat from pan juices. Combine 1/4 cup water and cornstarch, stirring until smooth. Add cornstarch mixture to pan juices, stirring with a wire whisk. Bring to a boil over medium heat, stirring constantly. Cook, stirring constantly, until thickened and bubbly. Serve gravy with lamb and vegetables. Garnish with rosemary sprigs, if desired.