1. Preheat oven to 400 degrees.
2. Place pumpkin cubes in one layer on a lined baking tray and pour over maple syrup and olive oil. Season with salt & pepper, then toss pumpkin to coat and bake for 20 minutes until tender. Reduce heat to 375 degrees.
3. Heat oil in a large heavy frying pan over low heat. Add the onions and salt and cook for 18-20 minutes, stirring occasionally.
4. Once the onions have softened and are golden, add the brown sugar and balsamic and continue to cook over low heat for 5-10 minutes until caramelized.
5. Roll out the rectangle of puff pastry. Transfer the pastry to baking paper and leaving a small gap around the edges, top evenly with the caramelized onion. Then top with slices of brie, the maple pumpkin, and add a few thyme leaves on top.
6. Transfer the tart and baking paper to a baking tray and bake for about 40-50 minutes until the pastry is golden. Remove from heat and set aside to cool before serving.