Drizzle some olive oil into a frying pan over a medium heat and cook the aubergine in batches for 5 to 7 minutes, or until softened and golden, adding a little more oil, if needed. Using a slotted spoon, transfer to a large bowl.
Peel and thinly slice the onion, and peel and slice the garlic. Add the onion and garlic to the empty pan, with a little more oil if necessary, and sauté for 5 minutes, or until softened. Transfer to the bowl with the aubergine.
Chop the potatoes into 1cm cubes, thinly slice the medium tomatoes and slice the courgettes.
Add the potato, all the tomatoes, the courgette, passata and 200ml water to the bowl. Sprinkle with the oregano, then pick, finely chop and add the parsley. Season generously with sea salt and black pepper.
Combine well with your hands and transfer to a large ovenproof dish. Drizzle with any remaining oil.
Bake for 30 minutes, then turn the oven down to 200°C/400°F/gas 6. Bake for another 20 to 30 minutes, or until the top has browned and vegetables are tender. Allow to cool slightly before serving.