1. Preheat oven to 400 degree F or 200 degree C.
2. Line a baking tray with parchment paper.
3. Place a heavy bottomed pan on medium flame, and heat oil in it.
4. Drop onion, stir and cook for 10 minutes or until golden brown.
5. Add balsamic vinegar and sugar. Cook for 2 minutes. Remove from heat and set aside to cool.
6. Cut the pastry sheet in half, and score around the outside edge.
7. Place the pastry sheet on the parchment lined tray.
8. Top the pastry with the Parmesan, leaving a 2cm border.
9. Spread onion, olives and oregano on top of Parmesan.
10. Pop the tray in oven, and bake for 20--25 minutes or until golden and puffed.
11. Serve onion and olive tart hot with a cup of coffee.