Season the turkey with salt and fresh ground black pepper. Heat oil in a large frying pan over medium high heat, brown the shanks turning until sides are golden brown.
Place the onion, potato and garlic into a large ovenproof dish, scatter over rosemary and half the oregano. Season with salt and pepper. Top with shanks, add olives and pour in lemon juice. Cover with lid and bake in 1600C preheated oven for 2 hours. Serve sprinkled with remaining oregano and rosemary sprigs.