Blanch, peel, core and quarter tomatoes removing bruised or discoloured portions.
Place 2 cups (500 ml) of tomato quarters in a large stainless steel saucepan. Bring to a boil while crushing and stirring to extract juice. Boil gently, stirring constantly; add remaining tomato quarters, 2 cups (500 ml) at a time. DO NOT crush tomatoes, as they will soften with heating and stirring. When all tomatoes are added, boil gently 5 minutes.
Place 1 tbsp (15 ml) lemon juice or 1/4 tsp citric acid and 1/2 tsp (2 ml) salt, if using, in hot 500 ml jar or 2 tbsp (30 ml) lemon juice or 1/2 tsp (2 ml) citric acid and 1 tsp (5 ml) salt, if using, in hot 1 L jar.
Ladle tomatoes into a hot jar to within 1/2 inch (1 cm) of top rim (headspace). Using nonmetallic utensil, remove air bubbles and adjust headspace, if required, by adding more tomatoes. Wipe jar rim removing any food residue. Centre hot sealing disc 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 tomatoes.
When canner is filled, ensure that all jars are covered by at least one inch (2.5 cm) of 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* for 500 ml; 45 minutes* for 1 L.
When processing time is complete, 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.
Borrowed from: https://www.bernardin.ca/recipes/en/crushed-tomatoes.htm