Using kitchen string, tie lamb at 2cm intervals to secure.
Using a large sharp knife, score top of lamb several times and place lamb in a glass dish and using hands rub harissa all over the lamb and cover with plastic food wrap and chill for 30 minutes to marinate.
Heat half the oil in a large frying pan over moderate heat and add lamb and cook for 3 minutes or until seared all over and transfer the lamb to a roasting pan and drizzle with 1 tablespoon of remaining oil and bak for 3 hours and 30 minutes or until very tender.
Remove from the oven and cover with foil and rest for 15 minutes.
Meanwhile place potato in a large microwave safe bowl and cover with plastic food wrap and microwave on high (100%) for 7-8 minutes or until tender, cool and place in a large bowl.
Juice 2 oranges and whisk remaining oil and orange juice, mustard and garlic in a small bowl and season with salt to taste.
Using a small knife, peel and segment remaining oranges and add to the potato and then add carrot, watercress, coriander and mint and drizzle with the orange dressing and toss to combine.