Preheat oven to 425 degrees.
Melt the butter in a medium skillet over medium heat; saute celery, onion and bell pepper until tender.
Add flour, salt, pepper, tarragon, basil and milk; blend well. Cook for 1 minute, stirring constantly until thickened.
Add the cheese and stir until melted. DO NOT BOIL!
Place fish in a baking dish and spoon the sauce evenly over the top. Bake for 8 to 10 minutes or until fish flakes easily when tested with a fork.