Preheat the oven to gas 7, 220°C, fan 200°C. Mix the peppers, onion, oil and garlic in a shallow roasting tin. Season well and roast for 15 minutes.
Meanwhile, unroll the pastry and cut into 4 rectangles. Score a 1cm (½in) border on each and prick the centre with a fork. Put on a baking sheet lined with nonstick baking paper and bake for 10 minutes.
Remove the pastry and the peppers from the oven, discard the garlic, and turn the oven down to gas 6, 200°C, fan 180°C. Flatten the middle of the pastry ready for the filling.
Divide the mozzarella between the tarts, spoon over the pesto, then top with the peppers, tomatoes, olives and pine nuts. Bake for 10-12 minutes, or until golden. Serve warm, topped with a few basil leaves.