1. Prepare outdoor grill with medium-hot coals, or heat gas grill to medium-hot, or heat oven broiler. Lightly coat grill rack or broiler-pan rack with nonstick cooking spray.
2. Saute onion in oil in nonstick skillet over medium-low heat to soften, 4 minutes. Remove from heat.
3. Squeeze excess moisture from zucchini. Combine zucchini with cooked onion, mushrooms, bread crumbs, Italian seasoning, garlic, egg and pizza sauce in bowl. Stir in cheese. Shape into 6 equal patties.
4. Grill or broil about 4 inches from heat, turning once, 10 minutes or until instant-read thermometer inserted in centers registers 160 degrees F. Serve on whole-wheat roll lined with watercress. Top with thick slices of tomato. Makes 6 burgers.