Put everything from the onion down to and including the black pepper in a large pot at least 8 litres (quarts) in size. Bring to a boil, then lower to a simmer and let simmer uncovered for 10 minutes.
Meanwhile, wash the tomatoes, core, and chop coarsely. Set aside.
Add tomatoes and any juice off them to the pot.
Bring pot back to a boil, reduce heat down to a simmer, and let simmer uncovered until both tomatoes and onion are very soft.
Press this tomato mixture in small batches through a food mill or sieve, and discard the skin and seeds.
Put strained tomato mixture back in the pot.
Add the basil.
Put heat to medium-high and let cook for about 45 minutes or until the volume of the sauce is reduced by half.
Remove basil.
Stir in either the citric acid OR the lemon juice.
Ladle sauce into heated jars.
Leave 2 cm (½ inch) headspace.
Debubble, adjust headspace.
Wipe jar rims.
Put lids on.
Process in a water bath or steam canner.
Process jars for 40 minutes; increase time as needed for your altitude.