Remove the dish from the oven and, using a fish slice or slotted spoon, transfer the fish fillets to a warmed serving dish. Keep warm while you make the sauce.
Strain the fish cooking liquid into a large measuring jug, pressing down on the vegetables and flavouring to extract all the liquid. Reserve 8 fluid ounces of the strained stock and discard the rest of the liquid and the contents of the strainer.
In a small saucepan, melt the butter over moderate heat. Remove the pan from the heat and add the flour, stirring constantly to make a smooth paste. Gradually add the reserved stock, stirring constantly. Return the pan to the heat, and cook, stirring for 3 minutes or until the sauce is thick and smooth. Stir in the cream and cook, stirring constantly, for a further 2 minutes or until the sauce is hot but not boiling.
Remove the pan from the heat and pour the sauce over the sole fillets. Serve at once.