1. Preheat the oven to 375°F (190°C). Lightly grease a baking dish with olive oil.

2. Place the thinly sliced sweet potatoes in a large bowl. Add the olive oil, salt, black pepper, and dried thyme. Toss gently until the slices are evenly coated.

3. In a small bowl, whisk together the heavy cream, minced garlic, and nutmeg until well combined.

4. Arrange half of the sweet potato slices in the prepared baking dish, slightly overlapping them. Sprinkle half of the crumbled feta evenly over the potatoes. Add another layer of sweet potatoes, followed by the remaining feta.

5. Pour the garlic and nutmeg cream mixture evenly over the sweet potatoes and feta. Sprinkle the shredded mozzarella over the top to create a golden, bubbling cheese layer.

6. Cover the dish loosely with foil and bake for 30 minutes. Remove the foil and continue baking for another 15–20 minutes, until the sweet potatoes are tender and the cheese is golden and bubbling.

7. While the gratin is baking, place the chopped walnuts in a dry skillet over medium-low heat. Toast for 2–3 minutes, stirring frequently. Add the honey or maple syrup, cinnamon, and sea salt. Stir continuously for another 1–2 minutes, until the walnuts are glossy and lightly caramelised. Transfer the walnuts to parchment paper and allow them to cool slightly.

8. Add the cranberry sauce, honey, balsamic vinegar, orange juice or water, and salt to a small saucepan. Warm over low heat for 3–4 minutes, stirring occasionally, until the mixture is smooth and glossy.

9. Remove the gratin from the oven and let it rest for 10 minutes. This allows the layers to settle and makes serving easier.

10. Scatter the caramelized walnuts generously over the warm gratin. Drizzle with the warm cranberry glaze and garnish with fresh thyme and freshly cracked black pepper. Add extra crumbled feta if desired and serve warm.