In a big stainless steel saucepan, combine the onion, celery, carrot and garlic with ONE CUP of the tomato purée. Cook on medium-high heat (stirring frequently) until it comes to a boil. Reduce heat, cover the saucepan, and cook this mixture for about 5 minutes (boiling gently). The vegetables should be tender at the end of this cooking time. While the liquid is still boiling steadily, add the rest of the tomato purée to the pan (one cup at a time), stirring well.

Add the bottled lemon juice, salt, black pepper and the dried red chili pepper flakes.

Turn the heat up to High. Bring the sauce to a rolling boil. Continue to boil the sauce hard, making sure you stir the sauce quite frequently. Continue boiling sauce until the mixture has been reduced by one-third (this will take about 15 minutes or so to reduce).

Ladle the tomato sauce (very hot) into the hot canning jars. Leave 1/2 inch headspace. Remove the air bubbles, and adjust the headspace in jars, if needed, by adding more sauce. Clean off the rim. Place the lid on the jar, and add the screw band; tighten screw band until fingertip tight.

Carefully place jars into simmering water in the canner. Make sure that hot water completely covers the jars and goes about an inch OVER the lids. Place lid on canner. Bring water to a full boil, then process jars for 35 minutes. When time is up, turn off heat and remove the lid from the canner. Wait for 5 minutes, then carefully remove hot jars of sauce (without tilting jars) to a dish towel on the countertop (do NOT place jars directly on countertop or they might crack). Let jars cool completely (24 hours), without disturbing them. Water on jar lids will evaporate naturally. After a few minutes, you should hear them make a "pinging" sound, indicating they have sealed properly. After jars have cooled for 24 hours and before storing in pantry, check to ensure lids have properly sealed, per USDA canning guidelines.