1. Place greens in a large, shallow bowl. Arrange grilled chicken and vegetables on top of greens. Scatter feta, chickpeas and olives on salad.
2. Combine balsamic vinegar and mustard in a small bowl. While whisking constantly, drizzle in olive oil in a steady stream. Season with salt and pepper. Toss salad with about 1/2 cup vinaigrette. Serve immediately, passing remaining vinaigrette on the side.