Instructions

• Heat the oven to 425°F. Line a baking sheet with parchment paper and set aside.
• Unfold the puff pastry and lightly roll once or twice on a lightly floured surface to remove the creases. Place on the prepared baking sheet, prick all over with a fork, and refrigerate until ready to use.
• Warm 3 Tbs. of the bacon fat or olive oil in a large skillet over medium heat. Add the onions, season with salt, and stir to coat. Add 3 of the thyme sprigs and cook, stirring frequently, until the onions are deeply caramelized, 30 to 35 minutes. If the skillet starts to burn, add 1 to 2 Tbs. water and scrape the bottom of the skillet to deglaze. Repeat as necessary. Remove the skillet from the heat and let cool.
• Put the mushrooms and the remaining 2 sprigs thyme in a large bowl. Drizzle with the remaining 3 Tbs. bacon fat or olive oil and season with salt. Toss to coat. Remove the pastry from the refrigerator and spread with onion jam, leaving a 1/2-inch border on all sides. Spoon the mushroom mixture on top of the onion jam (it will be a hefty mound) and transfer to the oven.
• Bake until the pastry is puffed and golden and the mushrooms are roasted on the edges, 18 to 20 minutes. Using a Microplane grater or the small holes of a box grater, grate the cauliflower over the tart. Drizzle with additional olive oil and sprinkle with salt and chives. Cut into pieces and serve warm or at room temperature.