Cut the leeks into 1 cm/½ inch dice if they are large or just cut them across at an angle if they are small. Wash and dry well.
Skin the fillet of brill and cut 12 slices across the fillet at a slight angle. Lay these in a buttered shallow tray or ovenproof pan, add the fish stock and the white wine and cover with buttered paper or foil. Heat the liquid until it just starts to tremble, then reduce the heat so that the liquid is not moving and allow the fish to poach for about 30 seconds. Remove the fish from the pan and keep warm.
Return the liquid to a high heat and reduce until it has almost gone. Add the cream, return to the boil, then add