Roughly chop the tomatoes. Gently fry the onion and garlic in the olive oil until soft and golden, add the tomatoes and cook until soft. Season the sauce with salt and pepper. Pass the tomato mixture through a vegetable mill.
When making this sauce it is a good idea to make extra and keep it in the deep freeze for use at a later date.
Always remember that summer tomatoes are best for this sauce so make a batch before winter sets in. It is a sauce used so often in Italian cooking that it is possible to buy a machine in Italy for puréeing the tomatoes from fresh. It is called a passa pomodoro, and is well worth buying if you should see one on a trip to Italy. The machine, which is not very expensive, also makes very good fruit drinks from peaches, pears, apricots etc.