Heat the cooking oil on medium-high in a medium size saucepan and add onions until soft. Add garlic and stir. Wait for 1 minute. (Don't use lid for this step, and can be generous with oil to avoid onions getting burnt)

Add passata, salt, sugar and put the lid on. Put 2 handfuls of water in the passata bottles, close and give it a shake and pour them into the saucepan. Turn the heat to medium-low. Put the timer on for 15 minutes. Stir twice between for even heat distribution. The sauce will become lighter in colour (red-orange), lose most of its tangy flavour and thicken a bit as the excess water cooks the sauce and evaporates. Turn off the hob.

Add chillies, sugar and black pepper and stir. Add rosemary and olive oil to the sauce, close the lid and let it rest for 5 minutes before serving. Taste before serving and add salt or sugar to adjust flavour if necessary.

When serving, first mix a little sauce with some pasta to make the pasta wet. Then pour a dollop of sauce over the pasta and eat when its warm. Goes really well with a balsamic vinegar based salad.