Thaw the fish if frozen. Arrange the fish in a 12 x 7 1/2 x 2 inch baking dish, turning under thin edges. Sprinkle with salt. Cook the mushrooms, onion, and tarragon in margarine until tender. Spoon over fish, sprinkle with paprika. Bake, covered in a 450ºF. oven for 6 to 10 minutes or until the fish flakes easily with a fork. Makes 4 servings