1.Heat oil in a large frying pan over medium heat; cook lamb until browned all over. Remove from pan.
2.Meanwhile, finely grate rind from lemon; reserve lemon. Combine garlic, rind, oregano and chopped dill in a small bowl. Rub mixture all over lamb.
3.Place potato over the base of a 4.5-litre (18-cup) slow cooker. Pour passata and stock over the potatoes. Top with lamb. Cook, covered, on low, for 8 hours.
4.Carefully remove lamb and potato from the cooker; shred the lamb coarsely using two forks. Cover to keep warm.
5.Pour cooking liquid into a medium saucepan; bring to the boil. Boil, uncovered, about 10 minutes or until liquid is reduced by half. Add olives; cook until heated through. Season to taste.
6.Cut reserved lemon into wedges. Serve lamb with potato, pan juices and lemon wedges; sprinkle with extra dill. Drizzle lamb with a little greek-style yoghurt before serving, if you like.
Ask the butcher to separate the shank from the shoulder, leaving it attached; this will help it fit into the cooker. We used dutch cream potatoes in this recipe, as they hold their shape well. Passata is sieved tomato puree available from supermarkets.