Preheat oven to 425° F (220° C). Break off eight 1-in (2.5 cm) pieces from rosemary sprigs; set aside. Set aside 8 garlic halves. Arrange onion, remaining rosemary and garlic in roasting pan, then top with lamb fat side up. Make eight 1-in (2.5 cm) deep cuts in lamb; stuff cuts with reserved garlic and rosemary. Season lamb with black pepper and rub with Knorr® Bovril® Liquid Chicken Bouillon. Roast 30 minutes.
Reduce oven temperature to 350° F (180° C) and roast an additional 1 hour 10 minutes or until meat thermometer reaches 145°F (63°C) for medium rare. Remove lamb to serving platter and keep warm.
Remove rosemary sprigs from pan and discard. Skim fat from pan drippings. Add wine and cook over medium heat, stirring occasionally, 5 minutes or until reduced slightly. Strain and return to pan. Combine flour and water in small cup; stir into pan and cook over medium heat, stirring constantly, until slightly thickened, about 2 minutes. Serve gravy with sliced lamb.