1.Combine lamb, garlic, juice, oil, oregano and half the thyme in large bowl. Cover; refrigerate 3 hours or overnight.
2.Preheat oven to 160°C/325°F.
3.Place lamb mixture in large baking dish; roast, uncovered, 3 hours.
4.Meanwhile, make skordalia. Boil, steam or microwave extra potato until tender; drain. Push potato through fine sieve into medium bowl; cool 10 minutes. Whisk combined garlic, juice, vinegar and the water into potato. Gradually whisk in oil in a thin, steady stream; continue whisking until skordalia thickens. Stir in about a tablespoon of warm water if skordalia becomes too thick.
5.Combine potatoes, rind, extra juice, extra oil and remaining thyme in large bowl. Place potatoes, in single layer, on oven tray. Roast potatoes,uncovered, for last 30 minutes of lamb cooking time.
6.Remove lamb from oven; cover to keep warm.
7.Increase oven to 220°C/425°F; roast potatoes a further 20 minutes or until browned lightly and cooked through.
8.Serve potatoes and lamb with skordalia and pan juices.