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. Drizzle with a little olive oil and season with salt, black pepper, and thyme. Roast for 12–15 minutes, until slightly tender but still firm enough to arrange on the tart.

3. Heat the remaining olive oil in a large skillet over medium-low heat. Add the sliced onions and season lightly with salt and black pepper. Cook for 25–30 minutes, stirring occasionally, until the onions are soft, deeply golden, and caramelised.

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

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

6. Place the thinly sliced Brie evenly over the sweet potatoes. Sprinkle with a little fresh thyme.

7. Bake for 20–25 minutes, until the puff pastry is beautifully puffed and golden 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. Reduce the heat and add the honey, cinnamon, sea salt, and red pepper flakes if using. Stir for another 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 walnut honey crumble generously over the warm Brie and sweet potatoes.

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