Step 1Rub lamb with garlic, 1/2 cup lemon juice, half the olive oil and half the oregano; place in a large shallow dish. Cover dish and refrigerate for 3 hours, or overnight (if time permits).
Step 2Preheat oven to 140°C. Place lamb in a large baking dish and roast, uncovered, for 4 hours.
Step 3Combine potatoes with lemon zest, thyme leaves, and remaining lemon juice and olive oil in a large bowl. Place potatoes on a baking tray in a single layer; roast for the last 40 minutes of lamb cooking time.
Step 4Remove lamb from oven and cover to keep warm. Increase oven temperature to 200°C. Roast potatoes for a further 20 minutes, or until tender.
Step 5Meanwhile, combine salad leaves, tomatoes, cucumber, onion, balsamic vinegar and remaining oregano in a bowl. Serve lamb with roast potatoes, salad and yoghurt on the side.