Brown Windsor soup? We know all about that.
As The Foods of England says:
Pick up pretty much any recent book on English food and you'll be told that Brown Windsor was The Victorian favourite, possibly the dominant English soup until WW2. You'll be told that it was always served at Windsor Castle, that it was the Queen-Empress's preferred starter, that it was a staple of boarding-houses and always turned up in railway dining cars. It is described as "the very soup reputed to have built the British Empire." and we're told that it "regularly appeared on state banquet menus". You'll learn, too, that it was thick and stodgy and that everybody hated it.That's the story we all know. But The Foods of England carries on:
All of which is very odd as we can't find any reference to it anywhere, scour though we have the cookery books, newspapers and literature of Victorian and Edwardian times. It isn't on menus, even railway ones, nor in magazines. It isn't in any novels, it isn't in encyclopedias and the National Archive have nothing on it. It isn't mentioned in any cookbooks, it isn't in Mrs Beeton, or Eliza Acton, and 'Punch ' doesn't even make fun of it. In fact this 'Victorian and Edwardian staple' doesn't turn up anywhere before the 1950's.Tellingly, Poirot's encounter with Brown Windsor soup was put in by the television scriptwriters. It is not mentioned in the original Agatha Christie story from the 1930s on which the programme was based.
The Foods of England also offers an explanation of this remarkably persistent myth:
Brown Windsor Soap, however, is well attested since the 1830's. Could it possibly be that the name was applied to ubiquitous hotel brown gravy soups as a joke, perhaps parodying the well-known rice-based White Windsor Soup?