Place the sliced onion in a pan with the extra virgin olive oil and soften over medium heat, being careful to lower the heat as soon as the onion browns.
Add the sliced peppers and stir fry them for 2 minutes, then cover the pan with a lid and cook over medium-low heat for 10 minutes.
Add the chopped canned tomatoes and their juice (optional), a few basil leaves and a ladle of warm water. Cover the pan with a lid, and cook over medium-low heat for 20 minutes.
Pour in the vinegar and sugar, season to taste, and cook all the ingredients for 2 minutes on high heat, to let the vinegar evaporate.
Remove the pan from the heat and transfer the peperonata to a serving plate. Top with more fresh basil and serve straight away or at room temperature.
