Depending on how fine you want the final product, and the kind of tomatoes you are using (Beefsteak, Roma, etc), you can slice them, cut them into cubes, or chop the tomatoes.
Peel and slice the onions. Don't worry about fancy - a food processor does a perfectly fine job.
In a large pot or bowl (I used a 42 cup bowl with a lid), sprinkle the tomatoes and onions well with salt, and let sit overnight covered. They do not have to be refrigerated.
In the morning, rinse the tomatoes and onions to remove the salt and drain the liquid off. Discard the brining liquid. The vegetables will have reduced considerably in volume.
Tie the spices in a cloth (cotton or cheesecloth) bag. This amount of spice will make a pickle with plenty of bite, and you can use much less and still have a good chow. One variation is to use 2 Tablespoons each celery seed and mustard seed, and put them directly in the pickle without a cloth bag.
Not everyone likes a strong vinegar taste. Replacing up to ⅓ of the vinegar with water will still work. Do not go less than that.
In a large pot, combine vegetables, vinegar (and water, if using), sugar, and spices.
Cook on medium heat for ABOUT 2 hours, stirring often.
The mixture will cook down and thicken.
Follow SAFE, up to date canning methods (defer to official recommendations over this recipe). Fill 6 pint jars with hot pickle, add canning lids and rings, and process in a boiling water bath for 15 minutes.
If any is left after filling the jars, place it in a container in the refrigerator and use within a week or so.
Let cool completely and test the lids. Any that do not seal should be refrigerated.
