Prepare the aubergines: Lightly spray a large non-stick frying pan with low-calorie cooking spray and heat over medium heat. Cook the aubergine slices in batches for 2-3 minutes per side until lightly golden and softened. Remove and set aside.
Make the tomato sauce: In the same pan, spray more cooking spray and sauté the onion for 3-4 minutes until soft. Add the garlic and cook for another 30 seconds.
Add the tomatoes and seasoning: Stir in the chopped tomatoes, tomato purée, oregano, basil, salt, black pepper, balsamic vinegar, and vegetable stock. Simmer uncovered for 10 minutes, stirring occasionally, until the sauce thickens.
Assemble the pan: Lower the heat and layer half the aubergine slices over the tomato sauce in the pan. Add half the mozzarella slices, then repeat with another layer of aubergine and mozzarella. Sprinkle the Parmesan on top.
Melt the cheese: Cover the pan with a lid and cook on low heat for 5 minutes, or until the cheese has melted and is bubbling.
Serve and enjoy! Delicious on its own or served with a side salad.