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1
Pour the olive oil into a wide, heavy pan and set over medium-low heat. Add the sliced onions and, if using, the garlic. Cook gently, stirring occasionally, until the onions are translucent and fully softened — they should offer no resistance to the spoon and carry no colour.
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2
Add the red and yellow pepper strips to the pan. Raise the heat very slightly and stir to coat them in the oil. Cook over medium-low heat, stirring from time to time, until the peppers have released their juices and begun to yield — they should be visibly limp and the pan liquid should have increased. This takes 15 to 20 minutes. Do not allow the peppers to brown; this is where the dish lives or dies: too high a heat now and the peppers caramelise hard rather than melting into silky ribbons, and no amount of subsequent cooking will recover their texture.
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3
Add the peeled, seeded, and chopped San Marzano tomatoes. Stir to combine. Reduce the heat to low and cook the assembly, uncovered, for 40 to 60 minutes, stirring occasionally. The vegetables are done when they have collapsed into a unified, cohesive mass — no pepper strip should hold a firm edge, and the liquid should be thick and reduced, not watery.
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4
Remove the pan from the heat. Tear the basil leaves and fold them through the stew. Taste for seasoning. The flavour should be sweet and savoury in equal measure, with the tomato's acidity keeping the natural pepper sugars in balance. Serve warm, at room temperature, or make a day or two in advance — the flavours marry and intensify markedly with rest.