For the sauce
Heat a saucepan over medium heat, add onion, season with salt and sauté until soft. Add garlic and as soon as it releases its aroma, add tomato paste and stir for about a minute. Add tomatoes, bring to the boil, reduce heat, season with salt, add sugar, if necessary, and simmer stirring frequently for approx. 1 hour. Add basil and cook for a further 5 minutes. Allow the sauce to cool and mix in a food mill or blender. Strain through a fine sieve. Cool rapidly and store in the fridge.