Take a medium-sized sole (or two, according to the number of people), remove the black skin and the head. Put the head or heads in a small saucepan with three mushrooms cut in slices, one onion, bouquet with thyme and bay leaf, salt and pepper, dry white wine and water. Bring to the boil, let it simmer till it has reduced by half and pour through muslin into a cup. Put aside the mushrooms, which will be used later in the dish. Put the soles in a buttered fireproof dish and pour over them the fish stock previously made, to which you add a little more white wine with a little tomato purée mixed with it. Cook slowly, basting often. When the soles are about half cooked, put in the mushrooms, sprinkle with breadcrumbs and go on cooking slowly, basting meanwhile till ready. The sauce should be a short one and the soles just slightly browned.