Cook the Spiced Apple Filling: In a medium saucepan over medium heat, melt the unsalted butter. Add finely diced apples, dark brown sugar, cinnamon, nutmeg, and lemon juice. Cook for 6-8 minutes, stirring frequently, until the apples soften slightly and release their sweet juices. Sprinkle in the flour (or cornstarch) and vanilla extract, cooking for 60 seconds until the sauce bubbles and thickens into a glossy glaze. Remove from heat and let cool completely (placing in the fridge for 15 minutes speeds this up).

Prep the Pastry: Preheat your oven to 400°F (200°C) and line two large baking sheets with parchment paper. Unroll each sheet of thawed, cold puff pastry onto a lightly floured surface. Gently roll with a rolling pin just to smooth any fold lines. Cut each square sheet into 4 equal quadrants, giving you 8 pastry squares total.

Fill & Fold: Lightly brush the perimeter edges of each pastry square with egg wash. Spoon 2 generous tablespoons of the cooled apple filling onto one diagonal half of each square, leaving a 1/2-inch border. Fold the opposite corner over the filling to form a triangle. Press edges together with your fingers to seal.

Crimp & Vent: Firmly crimp the sealed edges with the tines of a fork to ensure none of the bubbling juices leak during baking. Use a sharp paring knife to slice 2 small decorative vent slits on the top of each turnover so steam can escape freely.

Bake to Golden Flakiness: Transfer turnovers onto the prepared baking sheets, spaced 2 inches apart. Brush the tops generously with egg wash and sprinkle generously with coarse turbinado sugar. Bake at 400°F (200°C) for 20 to 25 minutes until puffed high, deeply golden brown, and shatteringly crisp. Transfer to a wire rack to cool for 10 minutes.

Glaze & Enjoy: Whisk together powdered sugar, milk, and pure vanilla extract in a small bowl until a thick, pourable white glaze forms. Drizzle generously in zigzag ribbons over the warm turnovers. Let the glaze set for 10 minutes, then bite in!