Whizz onions in batches in a food processor to a paste. Heat 1/2 cup olive oil in a very large pot and cook onions, covered over a medium heat.
Meanwhile, de-seed and chop capsicum (peppers) and puree in the food processor with the unseeded chillies in batches.
Add the pepper puree to the onion mixture along with the fish sauce or vinegar and sugar.
Simmer uncovered for about 3 hours, stirring every 15 minutes or so until thickened.
About 1/2 hour before it’s cooked. Remove a small portion and allow to cool. Taste and season, adding more chilli if it’s too mild. If you’ve using balsamic you’ll need lots of salt.
Divide hot jam between clean jars fresh from the dishwasher and seal with lids immediately.