Weigh the tomatoes, rinse them thoroughly under clean, running water, and cut them into quarters. Add them to a large prep bowl and set aside.

Remove the peeling from the onion. Chop and measure 1 cup of chopped onions.

Peel the garlic and mince it.

Rinse the basil leaves under cool water to remove dirt. Pat dry, then finely mince, measure 1/4 cup, and set aside.

Add the garlic and cook briefly until fragrant, then add the mixture to your saucepan.

Add the prepared tomatoes to the saucepan.

Bring the pot to a boil over medium-high heat. Once the tomatoes begin to soften and release their juices, reduce the heat to low and simmer uncovered for about 20 minutes,

Remove the mixture from the heat and let it cool slightly before straining.

Working in batches, run the cooked tomato mixture through a food strainer, food mill, or sieve to remove the skins and seeds, and smooth the sauce.

Return the strained tomato puree to the saucepan. Add the basil, and stir until combined.

Bring the sauce back to a simmer and continue cooking uncovered until it thickens and is reduced by about half its original volume, about 2 to 3 hours. Stir frequently for even cooking and to prevent sticking.

Wash your jars, lids, screw bands, and canning tools in hot, soapy water. Rinse thoroughly to remove all suds. Set aside to air dry on a clean kitchen towel.

Place the jar rack in the water bath canner, then add the jars upright in the canner and cover them with water. Bring the canner to a simmer (180˚F) for 10 minutes, and keep the jars hot until you are ready to fill them.

Spread a dry kitchen towel on the counter. Place the canning tools, lids, bands, citric acid or lemon juice, along with the measuring spoons, next to the towel.

Use your jar lifter to remove a jar from the canner, drain it, and place it on the towel. Keep the remaining jars in the canner to keep them hot.

Swirl your bubble popper through the jars to release air bubbles.

Wipe the rim of each jar with a damp towel. Center a lid on the jar, and screw on the band until it is fingertip tight.

Use the jar lifter to place the jar back into the canner, and repeat with the remaining jars.

Once all the jars are in the canner, adjust the water level to 2 inches above the tops of the jars.

Cover the canner and bring to a boil over high heat.

When the processing time is complete, turn off the heat, remove the cover, and let jars rest in the canner for 5 minutes.

Spread a kitchen towel on the counter, and remove the cover by tilting the lid away from you so that steam does not burn your face.

Use the jar lifter to carefully lift jars from the canner and place them on the towel. Keep the jars upright, and don't tighten bands or check the seals yet.

Let the jars sit undisturbed for 12 to 24 hours to cool and seal.

After the jars have cooled for at least 12 hours, check to be sure the lids have sealed by pushing on the center of the lid. The lid should not pop up. If the lid flexes up and down, it did not seal. Refrigerate the jar and use it within a few days.

Remove the screw on bands and wash the jars to remove residue. Label, date, and store your jars in a cool, dark place. Use within 12 to 18 months for the best flavor. Refrigerate the sauce once opened and consume within a few days.