In a saucepan, melt the butter and stir in the apples, brown sugar, cinnamon, lemon juice, and salt. Cook for 8–10 minutes, until the apples are soft. Whisk the cornstarch and water together, then stir it into the apples to thicken the sauce. Remove from heat and let cool.

Preheat the oven to 400°F. Roll out the thawed (but still cold) puff pastry on a floured surface to about 12” wide. Cut into 4–6 rectangles, depending on how large you’d like your pies.

Place the rectangles on a parchment-lined sheet pan. Spoon 2–3 tablespoons of filling onto one half of each rectangle. Fold over the pastry and press the edges with a fork to seal.

Whisk the egg with a splash of water, then brush over the tops. Cut small slits with a sharp knife to vent. Sprinkle generously with cinnamon-sugar.

Bake for 20–22 minutes, until golden brown and flaky. Let cool slightly before eating—the filling will be hot, just like the real McDonald’s pies!