Place halibut in shallow dish. Pour salad dressing over fish. Cover and refrigerate 1 hour, turning once. Remove fish from marinade, set aside. Coat grill rack with vegetable oil cooking spray. Place fish on grill 4-5 inches above medium-hot coals. Grill, brushing with marinade and turning once, just until fish begins to flake when tested with a fork, about 8-10 minutes on each side. To serve, cut fish into 6 serving-size pieces. Arrange 3 lettuce leaves on individual serving plates; place fish on lettuce. Sprinkle each with Parmesan cheese and croutons.
