Sauté the chopped onions on the oil over low heat for 3-4 minutes, until they become translucent but not browned.
Add garlic and sauté for another 30 seconds.
Add tomato purée (passata), water, and salt. Bring it to a boil over high heat. Then turn down the heat and cook for 30 minutes covered. Make sure it slowly simmers, stirring occasionally.
Optional: Cook for another 10 minutes uncovered if you want a thicker sauce.
Notes
Nutrition data only a rough estimate, per serving.