In a large non-stick or stainless steel frying pan, heat the 8 tablespoons of oil over a moderate heat. When the oil begins to warm up, add the thinly sliced onion and sweat it well with a tablespoon of water. Now add the chopped tomatoes and peppers (after having cleaned them of the seeds and white internal ribs), the sugar, a few basil leaves and season with salt. Put the lid on and boil the sauce for 10 minutes, stirring from time to time. Squash the tomatoes with a fork to release their juice, mix everything together well and cook for another 5 minutes, this time without the lid on the pan. If you like you can also add pepper or hot chilli pepper, for a spicier taste.