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 pastry into an 11-by-18-inch rectangle and cut into eight 4 1/2-by-5 1/2-inch rectangles.
Dividing evenly, place filling lengthwise on one half of each rectangle, leaving a 1/2-inch border. Combine 1 egg yolk with 1 teaspoon 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 (see below).
To serve, preheat oven to 400, with rack in lower third. Place desired number of frozen turnovers on a parchment-lined baking sheet. Combine 1 egg yolk with 1 teaspoon water; brush egg wash onto turnovers and sprinkle with coarse sugar (if using). Bake until turnovers are deep golden brown and juices are bubbling, 15 to 17 minutes, rotating sheet halfway through.