Combine bread crumbs, Parmesan, lemon zest, salt and pepper in a bowl. Add butter and toss to form a crumbly mixture.
In another bowl, combine Hellmann's® 1/2 the Fat mayonnaise-type dressing and tarragon. Arrange fish, skin-side-down, on a platter. Spread each fillet with mayonnaise mixture. Press crumb mixture onto each fillet.
Preheat barbecue on high for 10 mins. Reduce heat to medium-high. Arrange fillets on bbq skin-side-down. Lower lid and cook until fish flakes easily with fork and topping is nicely browned, about 10–12 mins.
Drizzle lemon juice over each fillet. Serve immediately.