Put the couscous into a larger bowl, bring the water with salt to a boil over high heat. Pour the liquid over the couscous, stir to separate any clumps. Cover with a lid, plate, or plastic wrap, set aside for 5 minutes then fluff with a fork.
In another bowl, mix lemon, dijon mustard, salt, pepper and olive oil to make the vinaigrette, add the couscous and the fresh veggies, mix well and set aside for 10 minutes.
Serve at room temperature or cool.