Serves: Makes 8 servings (as an appetizer or side dish)
- 1 pound (about 4) yellow onions, cut into 1⁄4-inch-thick lengthwise slices
- 3 tablespoons extra-virgin olive oil
- 1⁄4 teaspoon kosher salt
- 1 1⁄4 teaspoons fresh rosemary
- 1 sheet frozen puff pastry (9 ounces),thawed overnight in the fridge or at room temperature for 45 minutes (keep refrigerated until ready to use)
- 6 to 8 pitted kalamata olives, halved lengthwise
- 4 to 6 anchovy fillets (optional)
- Preheat the oven to 400°F.
- Line a baking sheet with parchment paper.
- With a chef's knife, cut the onions into 1⁄4-inch-thick lengthwise slices using the radial cut; you should have about 4 cups of onions.
- Heat the oil in a 12-inch skillet over medium heat. Add the onions and salt, and cook, stirring often and adjusting the heat as needed, until the onions are caramelized, about 35 minutes; they should be soft and darkened to a golden shade of tan. You may need to add a bit of water to the pan to prevent sticking. Add 1 teaspoon of the rosemary and season with salt to taste. Transfer the onions to a plate and let cool to lukewarm, at least 15 minutes.
- Meanwhile, prepare the dough. Gently unfold the sheet of puff pastry on a lightly floured surface. Mend any cracks in the seams by pinching the dough together. Roll out the dough to an 11-inch square. Lift it gently and set it on the prepared pan. Prick the dough with a fork in several places (but not the rim). Refrigerate the dough until you're ready to assemble the tart.
- Distribute the onions evenly over the dough (tuck in any strands that extend beyond the square, or they'll burn). Arrange the olives evenly on top. If using, lay the anchovies between the olives. Bake until the crust is golden on top and underneath all the way into the center (lift it up to check), 20 to 25 minutes.
- Transfer the tart to a cutting board. Sprinkle the remaining 1⁄4 teaspoon rosemary on the tart. Use a chef's knife to cut straight down through the crust and portion the tart into squares. Serve warm.