• 1 recipe Extra-Flaky Pastry
• 2 medium sweet potatoes (about 12 ounces each)
• 1 ½ tablespoons olive oil
• 2 large onions, sliced crosswise ¼ inch thick
• 1 ½ tablespoons balsamic vinegar
• ½ teaspoon granulated sugar
• ½ teaspoon kosher salt
• ²⁄3 cup plus ¼ cup crumbled Gorgonzola dolce cheese (or blue cheese of your choice)
• ²⁄3 cup sour cream
• ¹⁄3 cup chopped walnuts
• 1 egg white mixed with 1 tablespoon water
• 3 sprigs fresh thyme, plus more for garnish

Instructions

Preheat the oven to 400° and place a rack in the center. Line a large baking sheet with parchment or a silicone mat and set aside.

Make the Extra-Flaky Pastry and chill for 1 hour. Remove it from the fridge about 15 minutes before using to let it soften slightly.

Meanwhile, set the sweet potatoes directly on the center rack and roast until they are softened but still firm in the center, 35 to 45 minutes. When they are cool enough to handle, peel and slice into ¼-inch-thick disks.

While the potatoes roast, prepare the onions: Set a large skillet over medium heat and add the oil. Add the onions, turn to coat in the oil, then cover the skillet and reduce the heat to medium low. Let the onions wilt for 10 minutes, stirring once. Remove the cover, add the vinegar, sugar, and salt, and stir. Continue cooking the onions, stirring occasionally, until they are caramelized, 20 to 25 minutes more. Set aside.

In a small bowl, mash ²⁄3 cup Gorgonzola into the sour cream until mostly smooth. Set aside.

Shape the disk of slightly softened dough into a rough rectangle shape. On a well-floured work surface, roll the dough into a larger rectangle about 8 inches wide and 14 inches long. Roll the dough up around the pin and then unfurl it on the lined baking sheet. Spread the Gorgonzola mixture over the bottom of the crust, leaving a 2-inch border on all sides. Top with the onions. Lay the potato slices over the onions in three overlapping vertical rows. Sprinkle the walnuts over all. Fold the sides of the crust up over the filling, preserving the rectangle shape. Brush the edges with the egg white mixture. Strip the leaves from three thyme sprigs and sprinkle over the entire tart. Transfer to the oven and bake until the crust is golden brown and the filling is bubbly, 45 to 50 minutes. Let cool for 20 minutes, then sprinkle with the remaining Gorgonzola and garnish with some thyme sprigs. Serve warm or at room temperature.