Preheat oven to 450°. Line a 15x10x1-in. baking pan with foil; grease foil., In a large bowl, combine the first 5 ingredients. Add melted butter, garlic salt and pepper; toss to coat. Place vegetables in a single layer in prepared pan; bake until potatoes are tender, about 20 minutes., Remove from oven; preheat broiler. Arrange vegetables on 1 side of sheet pan. Add fish to other side. Sprinkle fillets with tarragon and salt; dot with softened butter. Broil 4-5 in. from heat until fish flakes easily with a fork, about 5 minutes. If desired, serve with lemon wedges and tartar sauce.