Instructions
Wash and slice tomatoes and onions. Place in a bowl, sprinkle with salt, and let stand 4 to 6 hours; drain.
Heat and stir sugar in vinegar until dissolved. Tie mustard seed, allspice, celery seed, and cloves in a spice bag. Add the bag, tomatoes, and onions to the vinegar. Add only the minimum water needed to cover.
Bring to a boil and simmer 30 minutes, stirring to prevent burning, until tomatoes are tender and transparent. Remove the spice bag.
Fill hot jars and cover with pickling liquid, leaving 1/2-inch headspace. Remove bubbles, adjust headspace, wipe rims, and apply lids.
Process pints for 10, 15, or 20 minutes at 0–1,000 ft, 1,001–6,000 ft, or above 6,000 ft. Process quarts for 15, 20, or 25 minutes at those elevations. Cool 12 to 24 hours and check seals.