Briefly blanch the chilli peppers in boiling water, cut them and the onions into very small cubes. Heat olive oil in a pan, add onions and chillies and simmer for 5 - 8 minutes over medium heat. Add tomatoes and simmer for another 15 minutes. Season to taste with salt, pepper and a pinch of sugar.
The salsa tastes best when it has been through for a day.