Place the filet in a baking dish, cover with white wine, cover the dish and poach the fish just until tender (do not overcook) in 325°F oven, about 15 – 20 minutes.
When fish is cooked, remove from pan and set aside, being careful not to break it into pieces.
Save the liquid.
In skillet, melt the butter and stir in the flour.
Add the fish liquid to this, mixing constantly so as not to have lumps.