Wash the peppers, cut them in quarters and remove the seeds and white internal membranes. Cut them into finger-wide strips and put them in a frying pan or shallow saucepan (non-stick if possible) with the chopped onion and tomatoes, the olive oil, salt and sugar (which removes the acidity from the tomatoes). Cover the pan and cook the Peperonata over a moderate heat for approximately 20 minutes. Stir delicately from time to time and crush the tomatoes ("only" the tomatoes, as soon as they begin to soften) slightly with a fork, to release their juice.
Taste your Peperonata to see when it is cooked and, if necessary, add salt. If the peppers should dry out but are still too “al dente”, add a little (hot) water to finish cooking them. Leave it to rest for 5-10 minutes before serving. Peperonata is also delicious warm or even cold. It’s an excellent side dish for grilled, roast or fried dishes, delicious used as a sauce on pasta or rice, and for preparing bruschettas.