Balsamic-Red Onion, Chard & Gorgonzola tart
This tart looks impressive, but really it comes together quickly and easily. Vary it by using other toppings, such as roasted fennel, tomatoes, and black olives or zucchini, feta, and thyme. The tart can be made a day ahead, refrigerated, and served at room temperature or rewarmed gently in a 350°F (180°C) oven. Cut it into smaller squares for a terrific party appetizer.
3 tablespoons olive oil
2 small red onions, halved and cut into 1-inch (2.5-cm) pieces
Kosher salt and freshly ground pepper
2 tablespoons balsamic vinegar
1 small bunch Swiss chard, stemmed and cut into 1-inch (2.5-cm) pieces
All-purpose flour for rolling
1 sheet frozen puff pastry (half of a 17 oz/ 530 g package), thawed
1⁄4 lb (125 g) Gorgonzola cheese, crumbled
1 large egg, lightly beaten
Preheat the oven to 400°F (200°C).
Warm 2 tablespoons of the olive oil in a frying pan over high heat. Add the onions and sauté until soft, about 4 minutes. Reduce the heat to low and season with salt and pepper. Cook slowly, stirring occasionally, until the onions turn a deep brown, about 10 minutes. Add the balsamic vinegar and cook until the liquid is absorbed, about 2 minutes longer. Transfer the onions to a bowl.
Return the pan to medium-high heat; do not wipe the pan clean. Warm the remaining 1 tablespoon olive oil and add the chard. Season with salt and pepper and sauté, tossing the chard to coat in the oil, just until beginning to wilt, about 3 minutes. Transfer to the bowl with the onions, toss to combine, and let cool.
On a floured work surface, roll out the puff pastry into a 10-by-14-inch (25-by-35-cm) rectangle. Fold over 1 inch (2.5 cm) of each side of the dough to create a border. Fold the dough gently in half and center it over a baking sheet lined with parchment paper. Gently unfold the dough onto the baking sheet.
Prick the dough all over with the tines of a fork. Distribute the chard and onion mixture evenly around the tart and top with the cheese, leaving the borders uncovered. Brush the borders of the tart with the egg. Bake until golden brown, 22–25 minutes. Let cool slightly, then cut into squares and serve.