Meanwhile, in a large bowl, stir together mustard, dry glaze, and oil. Set aside.
Slice chicken breasts in half. Core and quarter bell peppers. Slice zucchini thickly on the diagonal.
Place chicken, bell peppers, and zucchini in bowl with glaze. Stir evenly to coat. If you have extra time, let stand 10 min or cover and refrigerate overnight.
Place chicken and vegetables on grill. Barbecue with lid closed until chicken is cooked, about 6–8 min per side.
Remove grilled veggies as they’re cooked to a cutting board; coarsely chop and place in a bowl with salad greens. Divide between 4 dinner plates.
Thinly slice chicken; add to plates. Shake on toppings, if desired.