cut rockfish into serving-sized peices. Saute onion and celery in butter until tender. add mushrooms, salt, pepper, and tarragon. place fish fillets in bottom of buttered shallow baking dish. spoon mushroom mixture on top drizzle with lemon juice; sprinkle with cheese. bake at 450* allowing about 10 minutes cooking time per inch of thinkness or until rockfish flakes easily when tested with a fork.