Put the mint and lemon zest in a saucepan (reserve some small leaves for later) and cover with 600ml water. Bring to the boil, then add the couscous, cover and take off the heat.
Leave to stand for 5-10 mins until the couscous has absorbed all the liquid. Remove and discard the lemon zest and cooked mint. Top the couscous with the spring onions and reserved mint leaves, and serve.