Place the pitted fresh cherries into a deep, heavy-bottomed saucepan.

Add the granulated sugar and splash in about 2 tablespoons of water.

Heat the cherries over medium heat until they begin to break down and the sugar dissolves, about 5 to 8 minutes.

In a small bowl, stir together the cornstarch, lemon zest, and lemon juice until completely smooth, then pour this slurry into the simmering cherries.

Continue simmering until the cherry mixture begins to bubble rapidly up the sides of the pan.

Use a potato masher to gently crush the cherries directly in the syrupy sauce.

Continue simmering until the mixture thickens significantly and resembles a rich fruit jam.

Remove the pot from the heat and allow the filling to cool to room temperature.

Preheat the oven to 425°F and line a large baking sheet with parchment paper.

Place the thawed puff pastry dough onto a lightly floured board or clean work surface.

Roll out the dough into a 15x18-inch rectangle, achieving about a 1/8-inch thickness.

Cut the dough into four equal horizontal strips and five equal vertical strips to create 20 uniform squares.

Place a colander into a larger bowl and pour the cooled cherry mixture through it, using the potato masher to press out as much excess juice as possible. (Reserve this brilliant syrup for cocktails or serving).

Place exactly one tablespoon of the strained, thick cherry filling onto the center of each pastry square.

Fold the dough over the filling to form a triangle (or rectangle) and firmly seal the edges using the tines of a silver fork.

Transfer each sealed tart onto the prepared baking sheet.

Brush the top of each tart lightly with the prepared egg wash and sprinkle evenly with the coarse sugar.

Bake until the tarts are beautifully puffed, crisp, and golden brown, about 15 to 18 minutes.

Serve warm directly from the oven, or allow to cool completely.