Preheat oven to 400°F (200°C). Place the onion and oil in a frying pan over medium heat. Cook, stirring, for 10 minutes or until the onion is soft and turning golden. Add the balsamic and sugar and cook for 2 minutes, then set aside to cool. Cut the pastry in half (2 rectangles) and score around the outside edge. Then place on a tray lined with parchment paper. Top the pastry with the parmesan, leaving a 2cm border. Top with the onion, olives and oregano. Bake for 20–25 minutes or until the tart is golden and puffed. Serves 4–6.