Caramelized Onion, Apple and Cheddar Tart

Ingredients

• 1 (14- to 17-ounce) package frozen puff pastry (preferably all butter)
• 3 large yellow onions
• 4 tablespoons (1/2 stick) unsalted butter
• 6 ounces sharp cheddar cheese, preferably white (about 1 1/2 cups shredded)
• 10 to 12 sprigs fresh thyme
• 1 large egg
• 1/4 cup low-sodium vegetable or chicken broth, or water
• 1 teaspoon apple cider vinegar
• 1 teaspoon kosher salt, plus more as needed
• All purpose flour, for dusting
• 1 medium apple, such as Honeycrisp or Pink Lady
• 2 tablespoons honey, plus more for serving
• Red pepper flakes (optional)

Instructions

1. Thaw 1 package frozen puff pastry in the refrigerator according to package directions.
2. Thinly slice 3 large yellow onions (about 7 cups). Melt 4 tablespoons unsalted butter in a Dutch oven orheavy-bottomed pot over medium heat. Add the onions and stir them gently to coat with the melted butter. It will seemlike a lot, but they will cook down significantly.
3. Cook, checking on them every 5 minutes and stirring and scraping up any browned bits on the bottom with a wooden spoon,until deep brown, jammy, and caramelized, about 35 minutes total. If the onions start to get too dark, reduce the heat.
4. Meanwhile, line a rimmed baking sheet with parchment paper. Arrange a rack in the middle of the oven and heat the ovento 400°F. Grate 6 ounces sharp cheddar cheese on the large holes of a box grater (about 1 1/2 cups). Pick the leavesfrom 10 to 12 fresh thyme sprigs (about 1 1/4 teaspoons) and finely chop. Whisk 1 large egg in a small bowl with a forkuntil broken up.
5. When the onions are ready, add 1/4 cup low-sodium chicken or vegetable broth or water and scrape up any browned bitsfrom the bottom of the pot. Add 1 teaspoon apple cider vinegar and 1 teaspoon kosher salt and stir to combine. Tasteand season with more kosher salt as needed. Remove the pot from the heat.
6. Slide the parchment onto a work surface and lightly dust with all-purpose flour. Unfold the puff pastry onto theparchment (if your package has 2 sheets, place them side by side, slightly overlapping, so that they fit the bakingsheet). Using a floured rolling pin, roll it into a rectangle about the size of the baking sheet. Slide the puff,still on the parchment, onto the baking sheet.
7. Brush the edges with the egg to form a 1-inch border. Carefully roll about 1/2-inch of each side in on itselftoward the center to form the outer rim of the tart. Prick the pastry all over with the tines of a fork. Brush therim with the egg. Bake until light golden brown and slightly puffed, 10 to 15 minutes. Meanwhile, halve, core,and thinly slice 1 medium apple (2 cups).
8. Remove the baking sheet from the oven. If the pastry has puffed up, gently press down with a spoon to deflate.Transfer the onions onto the tart and spread into an even layer, stopping when you get to the folded edges. Sprinklewith half of the cheese. Arrange the apple slices evenly over the cheese. Sprinkle with the remaining cheese and thyme.Drizzle with 2 tablespoons honey, and sprinkle with red pepper flakes if desired.
9. Bake until the edges are puffed and the bottom is crisp and golden-brown, 18 to 30 minutes more. Let cool for at least5 minutes. Transfer onto a cutting board. Cut and serve warm with more honey if desired.