Pour the olive oil in to a large, heavy based saucepan and add the diced onion, capsicum, zucchini and eggplant.
Cover with a lid and cook over a low heat, stirring occasionally, for 12 minutes or until softened.
Add the minced garlic then place the lid back on and cook for another 20 minutes, stirring every so often to stop the vegetables from sticking to the bottom of the pan.
Add the chopped tomatoes and pinch of salt, turn the heat up to medium and cook for 5-7 minutes, until moist of the liquid has evaporated and the vegetables are tender and jammy.
Serve straight away, leave to cool to room temperature, or serve chilled.
Store any leftovers in the fridge, covered, for up to 3 days.
Notes
Vegetables - many traditional Spanish versions of this dish don't include zucchini (courgette) so feel free to leave it out if you wish. Storage - store in a covered container or in a bowl covered with cling film for up to 3 days. I do not recommend freezing this recipe.