In a small saucepan add vinegar, cinnamon stick, cloves and celery seeds.
Bring to a boil. Remove from heat and set aside.
Quarter and core tomatoes and put in a large kettle.
Add onions and cook on medium heat for 25 minutes or until tomatoes cook down.
Put tomato and onion mixture through a food mill or sieve and discard seeds and skins.
Return the pulp back to the kettle.
Measure the depth with a ruler.
If it is 2 inches high, then you want to cook it down by half, so in this example, you would cook it down to 1 inch.
Stir occasionally (This could take a little over an hour to accomplish).
Once it has cooked down, add your strained vinegar (discard spices), add salt and cook until you reach your desired thickness (about 30 more minutes). Keep in mind, the sauce will thicken as it cools down.
Add steviva blend or sweetener of choice and stir.