Fruit Turnovers

1/2 c sugar
2 Tbsp cornstarch
1/2 tsp coarse salt
3/4 pound fresh or frozen fruit, cut into 1-inch pieces if necessary
finely grated zest of 1 lemon
flour for work surface
2 sheets frozen puff pastry
2 large egg yolks
coarse sugar (optional)

In a medium bowl, combine sugar, cornstarch, and salt. Add fruit and lemon zest; toss to coat. On a lightly floured work surface, roll each sheet of puff pastry into an 11x18-inch rectangle and cut into eight 4-1/2x5-1/2-inch rectangles. Dividing evenly, place filing lengthwise on one half of each rectangle, leaving a 1/2-inch border. Combine 1 egg yolk with 1 Tbsp water. Brush border with egg wash; fold dough over filling and press edges to seal well. With the tip of a sharp knife, cut vents into pastry. Freeze turnovers on a parchment lined baking sheet until firm and layer with waxed paper in a resealable container. To serve: preheat oven to 400°F with rack in lower third. Place desired number of frozen turnovers on a parchment-lined baking sheet. Combine 1 egg yolk with 1 Tbsp water; brush over turnovers and sprinkle with coarse sugar. Bake until golden brown and juices are bubbling, 15 to 17minutes, rotating sheet halfway through.