Squash and Maple Caramelized Onion Tart

For the crust

• 250g plain flour
• 140g unsalted butter, chilled
• 4 tbsp water
• 1 tbsp maple syrup (preferably amber for its rich taste)

For the filling

• 4 tbsp extra-virgin olive oil, divided into two bowls
• 3 medium yellow onions, thinly sliced
• 3 cloves garlic, thinly sliced
• 2 tbsp Dijon mustard
• Pinch cayenne pepper
• 1 butternut squash, halved lengthwise and sliced
• 1 small, sweet potato, halved lengthwise and sliced
• 1 red onion, halved lengthwise and sliced
• 2 tbsp maple syrup (preferably golden colour for its delicate taste)
• Sea salt, to taste
• 5 sprigs thyme
• 2 tbsp unsalted butter

For the crust

1. In a large bowl, mix the plain flour and salt.
2. Add the chilled butter and work it into the dry ingredients with your fingers until combined.
3. Add the water and maple syrup.
4. Gently squeeze handfuls of the mixture together until a dough forms.
5. Pat the dough into a disk, wrap it in cling film and chill until very firm, this will take at least 3 hours.
6. Using a rolling pin, roll the dough out on a lightly floured surface to about ¼-inch thick.
7. Transfer the dough to a 12-inch fluted tart tin.
8. Gently press the dough into the corners and trim the excess.
9. Place the tart tin in the fridge for 15 to 20 minutes.

For the filling

1. Preheat the oven to 180c.
2. In a large skillet, heat 2 tablespoons of the olive oil, add the onions and cook, stirring occasionally, until the onions are soft and golden brown, this should take about 15 minutes.
3. Let the onions cool.
4. Brush the Dijon mustard over the chilled dough, and then sprinkle it with cayenne pepper.
5. Arrange the vegetables in concentric circles, starting from the outside edge and working your way inward, with the round edges standing up and the slices leaning against the crust.
6. Drizzle the remaining 2 tablespoons of olive oil and the maple syrup over the vegetables and season with sea salt.
7. Bake the tart until the edges of the vegetables are browned and the crust is golden brown, 45 to 60 minutes.
8. In a small saucepan, combine the thyme and unsalted butter and cook just until the butter is bubbling.
9. Brush the butter mixture over the tart with a pastry brush, sprinkle with salt flakes, and serve warm or at room temperature.