Place the chicken in a shallow dish. Whisk together lemon zest and juice, and olive oil, season, if you want, then pour over the chicken. Cover and leave to marinate for 10 mins in the fridge.
Meanwhile, put the couscous in a bowl and pour over the hot stock. Leave until the stock is absorbed, about 10 mins. Fluff up with a fork, cool for a few mins. Fold in the tomatoes, nuts and half the mint, then season to taste. Drain the lemony marinade from the chicken and mix two-thirds of it in with the couscous.
Pile the chicken on top of the couscous, drizzle over the remaining juices and top with the rest of the mint.