Put tomatoes and onions in a large deep stainless steel kettle or saucepan, sprinkle with salt, and let stand overnight.
In morning, drain well, pressing out as much water as possible. Return mixture to kettle. Add sugar, mustard, and spice bag. Stir in vinegar. Cook slowly, stirring frequently, until tomatoes are soft and clear-about 1 hour. Bottle in hot jars. Process in a boiling water canner.
Makes 7 or 8 pint-size (500 mL) jars.