Peel and finely chop the onion. Wash the green and red peppers, remove the cores and seeds, and dice into evenly sized pieces.
Wash the courgettes, trim the ends and dice into similar-sized cubes. Peel and finely chop the garlic cloves.
If using fresh tomatoes, bring a pan of water to the boil, blanch them for about 1 minute, then drain, peel, core and dice. If using tinned tomatoes, simply open the tin.
Heat the extra virgin olive oil in a large saucepan or deep frying pan over a medium heat.
Add the chopped onion and sauté for 5–7 minutes, until softened and translucent.
Stir in the diced peppers and cook for 10–12 minutes, stirring occasionally, until they start to soften.
Add the courgettes and garlic and cook for another 8–10 minutes, until the courgettes are tender but still hold their shape.
Stir in the tomatoes, season with salt and freshly ground black pepper, and mix well.
Bring to a gentle simmer, then reduce the heat to low and cook, uncovered, for 30–40 minutes, stirring occasionally, until the sauce has thickened.
Remove from the heat and let the pisto rest for a few minutes before serving.