This section is from the book "Preserving And Pickling", by Mary M. Wright. Also available from Amazon: Preserving and Pickling: Two Hundred Recipes for Preserves, Jellies, Jams, Marmalades, Pickles, Relishes and Other Good Things.
Arrange equal quantities of sliced onion, green sliced tomatoes and apples in alternate layers in an earthenware vessel, sprinkling salt liberally between layers, using about a tablespoonful to each three pounds of the mixture. Allow to stand overnight, then drain. Pass them through the fine cutter of the food chopper. Put them in a preserving kettle and add enough vinegar to cover, add spices and let simmer slowly until ingredients are cooked, and the consistency of catsup.
The apples may be omitted, and only onions and tomatoes used. Ripe tomatoes may be used instead of green if preferred.