Place 3 clean 500 ml mason jars on a rack in a boiling water canner. Cover jars with water and heat to a simmer (180°F/82°C). Set screw bands aside. Keep jars hot until ready to use. Preheating Bernardin® lids is not advised.

The sealing compound used for our home canning lids performs better at room temperature than it does pre-heated in simmering water (180°F). Simply wash lids in hot, soapy water, dry, and set aside until needed.

Preheating can lead to less vacuum being achieved during water bath canning, and to buckle failures during pressure canning. Blanch, peel, core and chop tomatoes; place in colander. Let stand 15 minutes; discard liquid. Purée tomato pulp in a food processor or pass through a sieve or food mill.

Measure 8 cups (2000 ml). Place onion, celery, carrot and garlic in a large stainless steel saucepan; add 1 cup (250 ml) tomato purée. Bring to a boil; reduce heat, boil gently, covered, until vegetables are tender, about 5 minutes.

Add remaining tomato purée 1 cup (250 ml) at a time while maintaining mixture at a boil, stirring frequently. Stir in lemon juice, salt, black pepper and red pepper flakes. Boil hard, stirring frequently, until desired consistency is reached, about 15 minutes.

Ladle sauce into a hot jar to within 1/2 inch (1 cm) of top rim (headspace). Using nonmetallic utensil, remove air bubbles. Wipe jar rim removing any food residue. Centre sealing lid on clean jar rim. Screw band down until resistance is met, then increase to fingertip tight.

Return filled jar to rack in canner. Repeat for remaining sauce. When canner is filled, ensure that all jars are covered by at least one inch (2.5 cm) of hot water. Cover canner and bring water to full rolling boil before starting to count processing time.

At altitudes up to 1000 ft (305 m), process - boil filled jars - 35 minutes. When processing time is complete, turn stove off, remove canner lid, wait 5 minutes, then remove jars without tilting and place them upright on a protected work surface.

Cool upright, undisturbed 24 hours; DO NOT RETIGHTEN screw bands. After cooling check jar seals. Sealed discs curve downward and do not move when pressed. Remove screw bands; wipe and dry bands and jars. Store screw bands separately or replace loosely on jars, as desired.

Label and store jars in a cool, dark place. For best quality, use home canned foods within one year.

Processing: At altitudes up to 1000 ft (305 m), process - boil filled jars - 35 minutes - 35 minutes.

Altitude-adjusted processing table: At altitudes up to 1000 ft (305 m), process - boil filled jars - 35 minutes