Green walnuts, pressed or crushed, steeped several days in vinegar, then boiled down with spices. A commonplace addition to gravies during the 18th and 19th Centuries.

Original Receipt in 'A New System Of Domestic Cookery' by 'A Lady' (Mrs. Maria Eliza Ketelby Rundell) (Rundell 1807);

WALNUT KETCHUP. - E. R.
Take two hundred walnuts at the season for pickling, beat them very small on a marble mortar, add about six handsful of salt; put them into a clean earthen pan, and stir them two or three times a day, for ten days, or a fortnight. Then strain them through a cloth, pressing them very dry. Then boil up the liquor with mace, cloves, sliced nutmeg, and whole pepper. When nearly done, add six cloves of shalot; bottle and cork it closely. The bottle should be shaken when the ketchup is used.


Original Receipt in 'The Art of Cookery Made Easy and Refined' by John Mollard, 1802 (Mollard 1802)

Walnut Ketchup for Fish Sauces.

TO a quart of walnut pickle add a quarter of a pound of anchovies and three gills of red port; boil them till reduced one third, strain it, and when cold preserve it in small bottles close corked.