Preheat the oven to 200°C.
Fry the chopped onion in olive oil with salt for 5 minutes until softened.
Add the garlic and cook for two additional minutes.
Stir in the canned tomatoes and simmer for 20-30 minutes until thickened.
Slice aubergines lengthways into 5mm thick slices and brush with oil.
Griddle or grill the aubergine slices until golden brown on both sides.
Spread a layer of tomato sauce in a 28x22cm baking dish.
Layer one third of the aubergines, sauce, and mozzarella thrice.
Bake for 25-30 minutes until the cheese is melted and bubbling.