Put the olive oil into a shallow pan over a medium heat and sauté the onion and garlic until golden and crispy. Add the courgette, yellow pepper and aubergine, and cook for 5–10 minutes until softened. Add the tomatoes to the pan with a little salt and pepper, then add the chopped basil. Allow the mixture to cool, then cover and chill until required.
Heat the vegetable oil in the same pan over a medium heat. Add the steaks and cook for about 2–4 minutes on each side, or according to taste. Season and remove from the pan.
When ready to serve, place the steaks on a baking sheet. Top each one with a quarter of the vegetables and a thick slice of mozzarella. Cook in a preheated oven, 200°C (400°F), Gas Mark 6, for 10–15 minutes until piping hot. Serve immediately.