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.
Slice without peeling one gallon of green tomatoes,-and peel and slice one quart of onions. Arrange in layers in a crock, placing salt between each layer, using about one cup of salt for this amount of tomatoes. Pour over one quart of water and let stand until morning, then drain off the water in the morning. Place in a saucepan or kettle one quart of vinegar, one cupful of water, two cupfuls of sugar, one teaspoonful each ground mustard and pepper, one-half teaspoonful each of allspice and cloves-or one tablespoonful of mixed spice may be used,-the spices tied up in a little bag and boiled in the vinegar.
Place the tomatoes in this and boil until tender, then fill jars.