1.Combine lamb with juice and half of the garlic in large bowl. Cover, refrigerate 3 hours or overnight, turning lamb occasionally in marinade.
2.Preheat oven to 180°C (160°C fan-forced).
3.Cut each potato into eight wedges. Place undrained lamb and potatoes in large flameproof baking dish, roast, uncovered, 1 hour.
4.Combine ingredients for gremolata in small bowl, press onto lamb. Roast, uncovered, further 30 minutes or until lamb is cooked as desired.
5.Remove lamb and potato from baking dish. Cover, keep warm.
6.Cook onion, celery and remaining garlic in baking dish, stirring, over heat until vegetables are soft. Stir in flour, cook, stirring, about 1 minute or until bubbling. Gradually stir in wine and stock, add rosemary, cook, stirring, until gravy thickens. Strain gravy into medium jug.
7.Serve lamb and potato wedges with gravy; sprinkle with parsley.