Begin by making the pastry. Place the flour and cold butter in a food processor and pulse until the mixture resembles coarse bread crumbs. Add the egg yolk and chilled water and pulse just until the dough begins to clump together into a ball. Turn it out onto plastic wrap, shape into a disc, wrap tightly and refrigerate for 30 minutes.

Divide the chilled pastry into 4 equal pieces. On a lightly floured bench or silicone baking mat, roll each piece into a circle roughly 8 inches (20cm) across and about ¼ inch (5mm) thick. Transfer the circles to a baking tray lined with parchment paper and refrigerate until needed.

Preheat the oven to 350°F (180°C). Toss the butternut pumpkin with the olive oil and season with salt and pepper. Spread it out in a single layer on a roasting pan lined with parchment paper and roast for 25 minutes, or until tender and lightly golden. Leave to cool for 5 to 10 minutes so it's warm, not hot. Leave the oven on.

Remove the pastry circles from the refrigerator. Spread a quarter of the caramelized onion relish over the center of each circle, leaving a border of about 2 inches (5cm). Divide the roasted pumpkin between the crostatas, piling it on top of the onion, and season with a little sea salt. Fold the pastry border up and over the edge of the filling, pleating as you go. Rub the thyme leaves between your fingers to release their scent and sprinkle over the top.

Bake for 20 to 25 minutes, or until the pastry is golden and crisp. Let the crostatas rest for 5 minutes, then serve warm or at room temperature.