A catsup in the eighteenth and nineteenth centuries was a source of piquancy for many dishes, a far more diverse seasoning than what passes under that name today. The word comes from an oriental term for the brine of pickled fish or shellfish; the Oxford English Dictionary defines it as “almost any salty extract of fish, shellfish, fruits, vegetables, or mushrooms.” The ubiquitous tomato sauce we know is not a fair representative of this large family of condiments.
Southern cooks would pickle unripened English or black walnuts for a walnut catsup. The nuts had to be green enough to be pricked through with a pin, and were put into a spiced brine, left to stand, and then strained out. Reduced and bottled, the liquid functioned in cooking much like a Worcestershire sauce. Many old southern recipes have been corrupted in later years by the modern cook’s ignorance of such sauces. Knowing only the tomato catsup, cooks have added it erroneously. Frequently, Worcestershire or an oriental oyster sauce would have been closer to the original intention. When you come across southern recipes that call for catsup, try imaginatively to reconstruct them historically. If tomato catsup seems at cross-purposes with the intended development of flavors, omit it.
A well-flavored catsup can be made from commercial mushrooms, but if you have access to chanterelles, flaps, or other foraged wild mushrooms, use them, by themselves or in combination. Shiitake mushrooms, now appearing in many markets, may be employed as well, though their high price will limit their use. Mushroom catsup as a flavoring is added to stews, soups, sauces, and gravies. In older southern cookbooks, this preparation is often referred to as mushroom soy, or simply, soy.