1. Preheat the oven to 400°F (200°C). Line a baking sheet with parchment paper.

2. Arrange the thinly sliced sweet potatoes on a baking sheet. Brush them lightly with olive oil and season with a little salt, black pepper and dried thyme. Roast for 12–15 minutes, until the slices are slightly tender but still firm enough to arrange neatly on the tart.

3. While the sweet potatoes roast, heat the olive oil and butter in a large skillet over medium-low heat. Add the sliced onions, salt and black pepper. Cook for 25–30 minutes, stirring occasionally, until the onions become soft, deeply golden and caramelised.

4. Place the thawed puff pastry on the parchment-lined baking sheet. Lightly score a border about 1 inch from the edge without cutting completely through the pastry. Prick the centre several times with a fork.

5. Spread the caramelised onions evenly over the centre of the puff pastry, keeping them inside the scored border. Arrange the roasted sweet potato slices over the onions, slightly overlapping them.

6. Arrange the thinly sliced Brie over the sweet potatoes. Scatter the dried cranberries evenly across the top.

7. Bake for 20–25 minutes, until the puff pastry is deeply golden and crisp and the Brie is soft and melted.

8. While the tart bakes, toast the chopped walnuts in a dry skillet over medium heat for 2–3 minutes, stirring frequently. Reduce the heat and add the honey, cinnamon, sea salt and red pepper flakes if using. Stir for 1–2 minutes, until the walnuts are glossy and lightly caramelised. Transfer the walnut mixture to parchment paper and allow it to cool slightly.

9. Remove the tart from the oven and scatter the warm walnut honey crunch over the melted Brie and roasted sweet potatoes.

10. Finish with fresh thyme leaves and freshly cracked black pepper. Add a light drizzle of balsamic glaze if desired. Slice and serve warm while the pastry is crisp and the Brie is soft and creamy.