Make the pesto by pulsing the basil, spinach and arugula mix, garlic, grated Parmesan, a pinch of salt, and some black pepper in a food processor until smooth, then slowly stream in the 3 tablespoons extra-virgin olive oil while pulsing until emulsified.
Prepare the couscous: place 1 cup couscous in a medium heat-proof bowl, add 1 1/3 cups boiling water and 1/4 teaspoon salt, cover, and let stand 5 minutes or until absorbed; fluff with a fork.
Stir 2 tablespoons of the pesto into the fluffed couscous, toss to combine, and set aside.
Pound the thicker ends of the chicken breasts to even thickness so they cook evenly; season both sides with 1/2 teaspoon salt and black pepper and lightly spray with olive oil cooking spray.
Spray the zucchini slices and halved tomatoes with olive oil cooking spray and season with salt and pepper.
Heat a grill or grill pan over medium-high heat. Grill zucchini in batches 2 to 3 minutes per side until tender, transferring finished slices to a baking sheet and covering to keep warm.
Grill the tomatoes 1 to 2 minutes, turning until slightly softened, then transfer to the baking sheet.
Grill the chicken 3 to 4 minutes per side until browned and cooked through (no pink in center), then transfer to a cutting board and slice.
Divide the pesto-seasoned couscous among 4 bowls (about 3/4 cup each), top with grilled zucchini, tomatoes, and sliced chicken, and drizzle with the remaining pesto before serving.