Heat the oven to 220°C. Roast the peppers for 15 minutes or until the skins bubble and blister, turning black in places. Remove carefully (hot juices can spill out from inside the vegetable) and put in a plastic bag or a folded cloth to steam. When cool, skin and remove the cores and seeds. Cut into strips.
Heat the olive oil and sweat the onions gently with a pinch of salt until tender and transparent. The onions should be almost caramelized but not burnt. Add the garlic and the bay leaf and cook a few minutes more. Add the peppers and tomatoes, season and cook, stirring, for 20–25 minutes. Scatter with the strips of basil.