In a medium saucepan, bring the stock, oil and a pinch of salt to a boil over high heat. Turn off the heat, stir in the couscous, cover and let steam until all the liquid is absorbed, about 5 minutes.
Fluff couscous with a fork, breaking up any clumps by rolling them between your fingers. Season to taste with salt, then stir in the parsley and lemon zest, if using.
