Place the required number of clean 500 ml or 1 L 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 sealing discs in hot water, not boiling (180°F/82°C). Keep jars and sealing discs hot until ready to use.
Wash and blanch tomatoes. Slip off skins; remove cores and any bruised or discoloured portions. Leave whole or halve.
Place tomatoes in a large stainless steel saucepan. Add just enough water to cover; bring to a boil; boil gently for 5 minutes.
Add quantity of lemon juice or citric acid specified below to each hot mason jar before packing tomatoes. If using, add salt to jar prior to filling.
Pack tomatoes into a hot jar to within 3/4 inch (2 cm) of top rim. Add hot cooking liquid to cover tomatoes 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 and hot liquid. 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 and hot liquid.
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-
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.
A great basic sauce to add to pastas, lasagnas and chilis. You can add more herbs and spice as desired but the basic tomato ratio to lemon juice should not be changed.