Wash the tomatoes and cut them into quarters. Heap the chopped tomatoes into a large pot and add the diced onion.

Add about a cup of water to the bottom of the pot to prevent scorching. Place the pot on the stove and bring it to a boil.

Cook, stirring occasionally for about an hour, until the tomatoes have lost their structural integrity and the pot contains nothing but super saucy tomatoes.

Remove the pot from the stove. Fit a food mill with its finest screen and position it over a large heatproof bowl.

Working in batches, start pushing the cooked tomatoes and onions through the food mill. You will probably need to stop three or four times to empty out the bowl into a clean pot.

Once all the tomatoes are milled, add the granulated garlic and Italian seasoning. Set the pot on the stove and bring to a low boil.

Cook for one to three hours, until the soup concentrate has reduced by at 1/3 and hopefully a bit more.

When you're pleased with the consistency, stir in the salt. Start with a tablespoon. Taste and add more as needed.

Divide the citric acid between five jars 26 ounce jars (the Ball brand 24 ounce Pint & Half jars are also a great choice for this one).

Funnel the finished soup concentrate into the jars, leaving 1/2 inch headspace.

Wipe the rims, apply the lids and rings, and process in a boiling water bath canner for 45 minutes (if you live above 1,000 feet in elevation, please adjust your processing times accordingly).

When the time is up, remove the jars and set them on a folded kitchen towel to cool. When the jars have cooled enough that you can comfortable handle them, check the seals. Sealed jars can be stored at room temperature for up to a year. Any unsealed jars should be refrigerated and used promptly.