Make a roux with butter and flour, and add enough liquid, stirring as you add it, to give the consistency of a white sauce.
Stir in capers.
Place sliced lemon, sliced onion, and celery on the bottom of a casserole tray deep enough to cover the entire fish.
Add the fish on top of the lemon, onion, and celery; cover with the wine and water.
Cover and poach gently for 30 minutes or until bass tests are done.
Arrange the bass on a platter and gently peel off visible skin.
Strain the cooking liquid and use it to make the caper sauce.
Spoon the sauce over the fish.