This relish will keep for over 12 months.

From 1922:

Cut up into small pieces 4 lb. ripe tomatoes and two large onions. Place them in an earthenware or china basin, and sprinkle over them a good handful of salt. Let them stand all night. In the morning drain off the liquid, put the tomatoes and onions into the preserving pan and add two cupfuls of vinegar. Bring to the boil, and boil for 5 minutes. Then add 1 ½ cupfuls of white sugar, 1 ½ table-spoonful of curry powder and 1 ½ table-spoonfuls of dry mustard mixed with a little cold vinegar, ¼ teaspoonful cayenne, a few peppercorns and any other spices liked. Boil from. 1 ½ to 2 hours. This relish will keep for over 12 months.