Meanwhile in a medium saucepan, bring 1 1/4 cups salted water to a boil. Stir in couscous; cover and remove from heat. Let stand until tender, 5 minutes. Fluff with a fork; set aside to cool, uncovered.
Make dressing: In a small bowl, whisk together lemon zest and juice and remaining tsp oil; season with salt and pepper.
In a large bowl, combine roasted vegetables with couscous, chickpeas and scallions. Place arugula on a serving platter, and drizzle with 1 tbsp dressing. Add remaining dressing to couscous mixture and toss; serve over arugula.