Line a roasting pan with a big piece of foil slightly longer than the fish, leaving ends hanging over. Smear middle section of foil lengthwise with 1/2 tablespoon soft butter and set fish on top. Smear top of fillets with remaining 1/2 tablespoon soft butter. Fold the sides of foil to the center and press against fish fillets. Twist both ends of foil to make a tight package. Bake for 30 minutes. Remove pan from oven and let fish rest 5 to 10 minutes, still in the foil package, while you make the sauce.