Heat the broth until boiling.
Heat the oil in a medium saucepan that comes with a lid, and add the couscous and fry, stirring all the time, for about 2-3 minutes.
Pour in the hot broth, still stirring, and keep stirring over a very gentle heat until the stock is absorbed into the couscous, about 5 minutes.
Turn off the heat, leaving the pan where it is, with a tight-fitting lid, for another 10 minutes.
(If you cook on gas, you could leave it on the lowest setting with a heat diffuser under the pan, if you have one.)
Fork through the couscous, turning it out into a big bowl.
Keep working the couscous with the fork to make it lump-free.
Sprinkle in the lemon zest and juice, sliced scallions, and salt and pepper, to taste, before adding the arugula leaves and tossing together carefully to mix.