Note: use a large oversized non reactive (stainless steel or enamelware) pot for stirring ingredients together because mixture will foam A LOT!
Put seven quarts of the tomato juice or puree that you get when you run your tomatoes through a food mill or strainer into the large oversized stockpot.
In blender, put the chopped onion, celery, flour, sugar, salt and pepper. Remove 2-3 cups of the puree from your stockpot and add to the ingredients in the blender and process until smooth and thick. You may have to do this in two or three batches depending on the size of your blender. It doesn't matter if you add the flour and ingredients in equal proportions, just so it all gets blended together and is smooth.
Add the thick liquid from the blender to the remaining tomato puree in the stockpot.
Cook over medium heat and stir constantly to avoid sticking. Cook until thick and bubbly and it comes to a boil.
Remove from heat and stir in baking soda gradually. Mixture will foam and foam, using a wire whisk helps stir it up and dissipate the foam. Stir slowly until it quits foaming, it takes a little while.
Bring to a boil again, stirring constantly. Boil one minute and then fill pint jars, adjust sterilized lids, and process for 30 minutes at 10 lbs pressure in pressure canner.
To serve: mix in equal parts of soup and water or milk.
Do not double this recipe, it does not turn out right, but if you don't like working with such large batches, you can cut it in half to make it easier to work with. Also you can make the juice/puree, measure it out and freeze it in gallon milk jugs, leaving 2" headspace, and then thaw it out and make your soup in the winter.