Brush the aubergines with 2 tablespoons of the oil and arrange, cut-side up, on a baking sheet. Roast in a preheated oven, 200°C (400°F), Gas Mark 6, for 20 minutes.
Meanwhile, heat the remaining oil in a frying pan, add the onion and garlic and cook until the onion is soft and starting to brown. Add the tomatoes and tomato purée and simmer for 5 minutes or until the sauce has thickened.
Remove the aubergines from the oven and cover each half with some sauce and 2 of the mozzarella slices. Season to taste with salt and pepper and return to the oven for 4–5 minutes to melt the cheese. Serve immediately scattered with basil leaves.