Scratch tomatoes crosswise and blanch with hot water so that the skin bursts. Quench the tomatoes cold, peel the skin and cut in half. Remove the seeds from the tomatoes with a spoon and remove the stalk.
Chop the tomatoes into small pieces.
Peel and finely chop the onions and garlic. Heat the oil in a pot. Fry the onions and garlic until transparent. Sprinkle with 2 tablespoons of sugar and caramelise slightly. Deglaze with vinegar, add tomatoes.
Simmer the sauce over medium heat for 20-30 minutes until slightly creamy, stirring frequently. Wash the basil, shake dry, chop the leaves and stir into the sauce. Season tomato sauce with salt, pepper and sugar.