What is "mushroom ketchup"

ketchup \ketch"up\, n. [Probably of East Indian origin, because it was originally a kind of East Indian pickles. Cf. also Malay k[e^]chap fish sauce.
--MW10.] A pureed table sauce made predominantly from tomatoes, flavored with onions, sugar, salt and spices; called also tomato ketchup. The term is also applied to pureed sauces containing mushrooms, walnuts, etc., being called in such cases mushroom ketchup, walnut ketchup, etc. [Written also catsup and catchup.]

Catchup \Catch"up\, Catsup \Cat"sup\, n. [Probably of East Indian origin, because it was originally a kind of East Indian pickles. Cf. also Malay k[e^]chap fish sauce.
--MW10.] A pureed table sauce made predominantly from tomatoes, flavored with onions, sugar, salt and spices; called also tomato ketchup. The term is also applied to pureed sauces containing mushrooms, walnuts, etc., being called in such cases mushroom ketchup, walnut ketchup, etc. [Written also ketchup.]

Mushroom ketchup is a style of ketchup (also spelled "catsup") that is prepared with mushrooms as its primary ingredient. Originally, ketchup in the United Kingdom was prepared with mushrooms as a primary ingredient, instead of tomato, the main ingredient in contemporary preparations. Historical preparations involved packing whole mushrooms into containers with salt. It is used as a condiment and may be used as an ingredient in the preparation of other sauces and other condiments. Several brands of mushroom ketchup were produced and marketed in the United Kingdom, some of which were exported to the United States, and Geo Watkins Mushroom Ketchup continues to exist in contemporary times as a commercially mass-produced product.