Mix tomatoes, pepper, onion and celery together in large pot.
Bring to a boil then lower to a simmer uncovered for 10 minutes.
Meanwhile to each half-litre (pint jar) you will be using add either ¼ teaspoon citric acid or 1 tablespoon bottled lemon juice; OR if using litre (quart) jars, add either ½ teaspoon citric acid or 2 tablespoons bottled lemon juice.
Pack tomato mixture hot into 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 ½ litre (US pint) jars for 40 minutes; process litre (US quart) jars for 50 minutes for altitudes up to 600 metres / 2000 feet. See table below.
Notes
Though it would be safe to season the mixture with dried seasonings such as salt, pepper, or dried herbs, it probably is best to leave it plain as the idea is for it to be a flexible base that you can take in a zillion directions with flavouring when you go to use it.The reason bottled (as opposed to fresh) lemon juice is called for is that it provides a guaranteed acidity level for safety.