Preheat your oven to 400°F (200°C). Line a large baking sheet with parchment paper and set aside.

In a medium skillet over medium heat, melt the butter. Add the diced apples, granulated sugar, cinnamon, and nutmeg. Cook for 5-7 minutes, stirring occasionally, until the apples are just tender and the mixture is fragrant.

Sprinkle the flour over the apple mixture and stir well to combine. Cook for another 1-2 minutes until the filling thickens slightly. Remove from heat and let cool for 10 minutes.

Unfold the thawed puff pastry on a lightly floured surface. Cut it into 9 equal squares using a sharp knife or pizza cutter.

Place the pastry squares on the prepared baking sheet, spacing them about 2 inches apart. Spoon 1-2 tablespoons of the cooled apple filling into the center of each square, leaving a 1/2-inch border around the edges.

Brush the exposed pastry edges with the egg wash, being careful not to let it drip onto the baking sheet as this can prevent rising. Sprinkle the coarse sugar generously over the egg-washed edges and filling.

Bake for 18-22 minutes, until the pastry is deeply golden brown and puffed around the edges. The bottoms should be crisp and golden when you lift one gently with a spatula.

Let the pastries cool on the baking sheet for 10 minutes. Meanwhile, whisk together the powdered sugar and 1 tablespoon of milk in a small bowl, adding more milk as needed to reach a drizzle consistency. Drizzle the glaze over the warm pastries before serving.