To Prepare Mason Jars:

• Wash mason jars and screw lids in soapy water and rinse soap off well under running hot water. Place clean jars on a baking sheet and place in oven preheated to 200 degrees. Set screw bands aside. Next, boil a kettle of water and pour into a clean glass bowl. Carefully submerge the sealing discs in the bowl of hot water. Set aside.
• Next, peel and quarter the onions. Peel and roughly chop the garlic. Add the onions and garlic to a food processor and pulse until they form a very fine puree. You may have to do this in batches, depending on the size of your food processor. Transfer the pureed onion and garlic to a large stock pot.
• Add the olive oil to the pot and cook the onion and garlic mixture over medium heat for five minutes. In the meantime, quarter the peeled tomatoes and cut away the stem. Puree the tomatoes in batches as well. Once pureed, add them to the cooked onions and garlic. Once all of the pureed tomatoes are in the pot, add the salt and stir well. Cook the tomato sauce for two hours, stirring often. While the sauce cooks, prepare the jars and your canner.
• Remove the sterilized jars from the oven. Add one teaspoon of bottled lemon juice to each jar. Ladle the sauce into prepared mason jars using a funnel to prevent the mixture from touching the rim of the jars.
• Wipe down the rim of each jar with a damp paper towel to ensure none of the sauce has come in contact with the rim.
• Carefully remove the sealing discs from the hot water with a magnetic lid lifter. Position the sealing disc directly onto the lid of the jars. Do not touch the underside of the lid.
• Screw on the screw bands until firm - do not apply pressure! Just use your fingertips to tighten the screw bands.
• Using the jar lifter, place the jars into the water bath canner with the boiling water. Place a lid on the pot and boil the jars for 20 minutes.
• Once done, carefully remove each jar from the canner using the jar lifter. Do not tilt the jars or try to wipe them dry. Place jars onto a wire cooling rack that has been covered with a clean kitchen towel.
• Leave the jars to cool for a minimum of 12 hours. Once cooled, wipe the jars of any residue that might have been transferred to the outside of the jar during the boiling process. Label the jars and store in a dark, cool cabinet.