Heat 1 tablespoon of the oil in a large frying pan over a medium heat, add the onion and cook for 5–7 minutes until softened. Stir in the garlic and cook for a further 1 minute. Add the tomatoes and a little water and simmer for 15 minutes.
Meanwhile, heat 1 tablespoon of the oil in a separate frying pan, add the green pepper and cook for 5 minutes, stirring, until softened and lightly browned. Remove from the pan and set aside. Add another 2 tablespoons of the oil to the pan, add the aubergine and cook for 5 minutes until golden, then set aside. Add the remaining oil to the pan, add the courgette and cook for 5 minutes until golden.
Return the pepper and aubergine to the pan, pour over the tomato sauce and season well. Bring to the boil, then reduce the heat and simmer for 10 minutes until the vegetables are very tender and most of the liquid has evaporated.