Chop tomatoes up in quarters and put them into the vegetable blender accompanied with the basil, oregano and some salt. In case of pureed tomatoes just mix the ingredients together.
Blend them mildly.
Pour some oil into a saucepan, heat it up then add the diced onions.
Saute on high heat until the onion gets a translucent / glassy look for about 3 - 5 minutes.
Reduce the heat and slowly cook the onion while stirring every now and then until it starts to caramelize for about 15-20 minutes.
Pour blended tomatoes into the saucepan and bring it to boil.
When it bubbles turn the hob on low heat and leave the sauce there to be cooked for about half an hour. Stir it occasionally.
When its consistency becomes thick, take it off the heat.
Use it immediately or save it for later by storing it in a jar in the fridge. It also freezes well.