Turbot a la Creme

From 1896:

Here is a recipe which an American epicure purchased from a chef in Paris for the sum of $500. It ought to be good, you will say, and it is, excellent.
Pour one pint of milk into a double boiler, and place in it a small bag containing one small onion, a teaspoonful each of salt, pepper, and Bell’s sausage dressing. Tie the bag close and very tight around its contents so as not to discolor the milk. Let this come to a boil. Beat together one egg and a tablespoonful of flour, remove milk from stove and stir in egg and flour, first removing the seasoning bag. Add also half a cup of butter. If the mixture is not smooth, strain it.
Pick into pieces fragments of fish, already cooked, and arrange in a baking dish for the oven, alternating the fish with layers of cracker or breadcrumbs, turning some of the custard over each two layers of fish and crumbs. Bake thirty minutes in a moderate oven.