Preheat the oven to 200C/400F/Gas 6.
Heat a saucepan over a medium heat. Add one tablespoon of the olive oil and, once hot, add the onion and a pinch of salt. Fry the onion for 4-5 minutes, or until softened. Add the garlic and continue to cook for two minutes.
Pour in the tomatoes and mix well, breaking up any larger bits with the back of your spoon. Season with a pinch of salt and pepper and bring to a simmer. Reduce the heat and simmer gently for 20-30 minutes, or until the tomato sauce is thickened and flavoursome.
Meanwhile slice the aubergines lengthways into slices 5mm/¼in thick. Brush the aubergine slices with olive oil and season well with salt and pepper on both sides. Heat a griddle pan until smoking hot and then griddle in batches for 2-3 minutes on each side, or until all the slices are golden brown. Alternatively, heat the grill to high, place half of the aubergine slices on a baking tray and grill for 3-4 minutes on each side. Continue with the rest of the slices until they are all golden-brown. Remove and leave to one side once cooked.
Add a spoonful of tomato sauce to the bottom of a medium sized ovenproof baking dish (about 28x22cm/11x8½in)and spread evenly. Top with a third of the aubergine slices.
Follow with a third of the remaining tomato sauce and top with a third of the mozzarella. Repeat the process for another two layers finishing with tomato sauce and the mozzarella. Place into the preheated oven and bake for 25-30 minutes, or until the dish is bubbling and the mozzarella melted and golden-brown on top.