Have some soles filleted; put the heads and bones into a flat saucepan with one onion and one carrot cut in slices, thyme, bay leaf, parsley, tarragon, salt and pepper, and a tumblerful of white wine and water mixed in equal parts. Bring to the boil and keep it boiling gently for ten minutes; then poach your fillets in this stock, remove them and keep them hot, after which you go on reducing this stock till there are only about two tablespoonfuls left. Pass it through muslin into another pan, add a tumblerful of fresh cream, a pinch of paprika, and boil again, stirring well, till it is reduced to the proper consistency, which should be that of thick cream. Add, just before finishing, a little chopped tarragon and, just before serving, two or three pieces of fresh butter. Pour over the fillets of sole in the serving dish.
I must insist on the fact that the thickening can only be obtained by the boiling and the reducing of the cream; it cannot fail to happen. On the other hand, any attempt at thickening with flour, or trying to make an ersatz cream sauce with a foundation of béchamel, would spoil this dish altogether.