Heat the oven to 200°C/Fan 180°C/Gas 6. Finely grate or pare the zest from the orange. Spread it out on a baking tray. Bake for 5 mins. Sprinkle over 1 tsp of cumin seeds. Bake for another 5 mins.
Tip the orange zest and cumin into a pestle and mortar or spice grinder. Grind them to make a fine powder. Add a pinch of harissa and a glug of olive oil. Stir to mix. Rub the paste into the lamb. Put to one side. Fill a kettle and boil it.
Peel and slice the carrots. Pop them in a pan. Fill with hot water from the kettle. Cover. Bring to the boil. Boil for 2-3 mins till just starting to soften. Drain and pat dry with kitchen paper. Lay out on the baking tray in a single layer. Drizzle with olive oil. Sprinkle over 1 tsp of the cumin seeds. Season. Roast for 10 mins.
While the carrots cook, sprinkle the lamb with a little salt. Heat a glug of olive oil in a frying pan. Add the lamb. Fry for 1-2 mins till golden brown. Turn over. Fry for another 1-2 mins.
Pop the lamb in the roasting tin with the carrots. Roast for 5-10 mins. Take the tin out of the oven. Cover with foil. Leave the lamb and carrots to rest for 5-10 mins.
Pop the couscous in a heatproof bowl. Pour in 150ml hot water. Cover with cling film or a saucer. Put to one side to soak. Peel and slice the onion thinly. Rinse and roughly chop the watercress. Rinse and chop the parsley. Halve the olives.
Fluff the couscous with a fork. Stir in the roasted carrots from the lamb tray, the sliced onion, watercress, parsley and olives. Taste and adjust the seasoning. Slice the lamb and serve with the couscous.