A scallion tart -- flavored with chiles, olives, walnuts, and Parmesan -- can be made for a crowd or as individual tarts.
Preheat oven to 400 degrees. On a lightly floured work surface, roll out pastry sheets just to smooth creases, and trim edges slightly to make even.
Cut each sheet into four squares; place on a parchment-paper-lined baking sheet, 2 inches apart. Freeze 20 minutes.
Place scallions, garlic, chile, walnuts, olives, and oil in a medium bowl. Season with salt and pepper; toss. Set aside.
In a small bowl, whisk together egg yolk and 1 teaspoon water. Brush a 1/2-inch border around edges of dough squares with egg wash. Divide scallion mixture among squares, leaving a 1/4-inch border; sprinkle square with cheese.
Bake until crust is golden brown, about 20 minutes. Transfer to a wire rack. Serve warm or at room temperature.
Baked tartlets can be frozen up to 3 weeks. Without thawing, reheat them in a 350 degree oven for about ten minutes. Instead of tartlets, you can make two large tarts: Cut pastry into two eight-inch squares; bake for about thirty minutes.