Puff Pastry:

1. In a large bowl, roughly cut 225g unsalted butter into 225g all-purpose flour using a pastry blender. Sprinkle in 1/2 tsp salt.
2. Slowly drizzle in 1/2 cup cold water, mixing with a wooden spoon until the dough comes together. Cover with plastic wrap and chill for 15 minutes.
3. On a floured surface, roll the dough into a rectangle about 1/4 inch thick. Trim edges for straighter lines.
4. Fold the top edge down the center, then fold the bottom edge to meet it.
5. Fold the whole rectangle in half like a book. Wrap in plastic and chill 20 minutes.
6. Repeat rolling and folding 3 more times (total of 4 book folds), chilling 20 minutes between folds.
7. Wrap the dough and refrigerate it overnight, ready to use the next day.

Strudel:

1. Preheat oven to 400°F (200°C) and line a baking tray with parchment paper.
2. Toss 2 peeled, cored, thinly sliced apples with 1/4 cup sugar and 1/4 tsp cinnamon. Set aside.
3. Roll 1 sheet of puff pastry to a 10-inch square on a lightly floured surface.
4. Cut a 4×10-inch rectangle and place it on the prepared tray.
5. Brush with egg wash (1 egg whisked with 2 tbsp water).
6. Cut a second rectangle, 4½x10-inch, and return it to the fridge to keep cold.
7. Cut remaining pastry into 1/2-inch strips.
8. Place strips along the outer edges of the base rectangle.
9. Brush with egg wash; this frames the apple filling.
10. Pull the top pastry from the fridge and cut 1/2-inch slits at intervals.
11. Lay it over the apples, brush with egg wash, and sprinkle with turbinado sugar.
12. Bake at 400°F (200°C) for 10 minutes, then lower oven to 375°F (190°C) and bake 20–25 minutes more until golden brown.
13. Serve warm or at room temperature with a drizzle of caramel if desired.