Place all dressing ingredients in a blender, then purée until very smooth. Transfer to a bowl then set aside.
If you have a grill basket, you can cut the zucchini, squash, and red onion. If not, you’ll want to grill them in large pieces and then cut. I’ll include both directions below.
Bring water to a rapid boil over high heat. Add dry couscous and kosher salt, then return to a boil. Immediately reduce the temperature to a simmer, then cover the pan and cook. Remove the lid, then use a fork to fluff the couscous.
Add the cooked couscous directly on top of the grilled veggies. Add baby arugula, then pour the basil vinaigrette on top. Sprinkle with most of the chives. Taste, adjust seasoning as needed. Enjoy hot, room temperature, or cold!