Slice the aubergines, season with salt and drizzle the olive oil over.
Arrange the slices on a baking tray lined with non-stick paper, and bake for 20 minutes at 200 degrees Celsius (390 Fahrenheit) until they are tender.
Meanwhile, make the tomato sauce.
Heat up the vegetable oil on a low to medium heat, peel and chop the onion finely, and fry until golden.
Add the tomato passata, season with salt and pepper, and leave to bubble up for 5 minutes until the sauce is slightly reduced.
Remove from the heat and set aside.
When the slices are cooked, add 2 tablespoons of tomato sauce to the bottom of an ovenproof dish, and arrange 4 aubergine slices in a single layer (or less slices depending on the size of your dish).
Sprinkle ⅓ of the grated parmesan over, tear the mozzarella cheese, and add ⅓ of it to the parmesan.
Add another layer of aubergine slices, and repeat the process until all the ingredients are used up - I had 3 layers of aubergines, sauce, parmesan and mozzarella.
Cover the dish with a kitchen foil, and bake for 20 minutes, then remove the foil and bake for a further 15 minutes until the cheese is melted and the dish is bubbling hot.
Garnish with freshly-shopped basil.