2 tablespoons olive oil
4 medium onions, chopped
1⁄4 cup sherry
3 egg yolks
1⁄2 cup heavy cream
3 ounces blue cheese, crumbled
2 tablespoons fresh thyme, finely chopped
1⁄2 teaspoon freshly ground black pepper
1 package (about 1 pound) all-butter puff pastry, defrosted
1. Caramelize: Heat oil in a large heavy skillet over low heat. Add onions and cook slowly, stirring occasionally, until deep brown and sticky sweet, about 1 hour. Turn up the heat, add sherry, and deglaze the pan, scraping any browned bits off the bottom. Cool.
2. Mix: Beat 2 of the yolks briefly in a small bowl, then stir in cream, cheese, thyme, and pepper. Stir in onions. (If you’re working ahead, transfer to an air-tight container and refrigerate until you’re ready to bake the tart.)
3. Bake: Unfold the puff pastry onto a baking sheet. Score with a pastry wheel into 3-inch squares. Separate slightly. Paint the pastry with the remaining egg yolk, lightly beaten. Slide into a 375-degree oven for 10 minutes until pastry begins to puff. Remove from oven. Spoon the onion lling onto each square, leav- ing a 1⁄2-inch bare border. Return to oven and bake until fully puffed, about 30 minutes. Serve warm.