Wash and cut the tomatoes and onions into 1/8-inch slices. Place in a large bowl and sprinkle with the salt. Cover with a large plate and place a heavy weight, such as a clean stone, on top of the plate to hold it down. Let stand overnight and wash and drain the tomato mixture 3 times on cold water. Place in a kettle and add the vinegar and sugar. Tie the mixed pickling spices in a cheesecloth bag and add. Simmer, uncovered, until the tomatoes and onions are tender, about 5 minutes. After removing the spice bag, pour at once into clean, hot, sterilized jars and seal as the jar or lid manufacturer directs.