Simmer the tomatoes and onions uncovered until they are reduced to a bit less than ½ in volume. (About 20 minutes.) Then press through a sieve or a food mill.
You will end up with tomato sauce equalling about 2 litres / 8 cups / 64 oz. If you have lots more than that, your variety of tomatoes may have had a lot of water in them -- keep on simmering the sauce to get it down to roughly around this amount. (You may wish to do up till here the day before, then refrigerate overnight, and bring back up to a boil the next day and proceed.)
Tie the cloves, cinnamon, allspice berries and celery seed in a cloth, put in a large measuring jug with the vinegar, nuke in microwave for 4 minutes.
Remove carefully from microwave, watching for liquid surge. Stir, then remove and discard spice bag.
Add vinegar along with cayenne pepper, mustard powder, garlic powder, either sugar OR liquid stevia, and either salt OR salt sub to the tomato sauce. Bring to a boil, lower right away to a simmer, let simmer slowly uncovered until the ketchup reaches the thickness you desire on a spoon. It should be able to form a mound. This could take several hours.
Fill each heated jar with the ketchup mixture up to 2 cm (½ inch) from the top.
Debubble, adjust headspace.
Wipe jar rims.
Put lids on.
Process in a water bath or steam canner.
Process either size jars for 15 minutes; increase time as needed for your altitude.
You can use a food processor to finely chop the onion.Yes, you must peel the tomatoes to reduce the bacterial count for safe canning: most of the bacteria is on the skin.How long the simmering to thicken it will take varies wildly. The broader the pot, the faster it will reduce.