Wipe the fillets with a clean, damp cloth. Put them flat in a shallow pan with the sliced mushrooms, sliced shallot, parsley and seasoning. Add the wine, cover and poach for 10–15 min. Drain the fish from the wine, place on a fireproof dish and keep warm. Boil the wine rapidly until it is reduced by half, then stir it into the hot Velouté sauce and thicken with a little butter. When thoroughly blended pour the sauce over the fillets and place under a hot grill until lightly browned. Serve at once in a border of sliced, steamed potatoes.