Set out puff pastry sheet to thaw - you want it to still be cold, but no longer frozen.
Preheat oven to 400° F. Prepare a baking sheet with parchment paper or a silicone baking mat and set aside.
In a large bowl, squeeze the juice of one lemon (making sure to remove any seeds).
Peel and thinly slice your apples, placing them in the bowl of lemon juice and coating thoroughly as you add them to the bowl.
In a small bowl, whisk to combine almonds, sugar, cinnamon, and nutmeg.
Add dry mixture to bowl of apples and gently toss (and/or stir) to coat. Add flour and stir to coat.
Roll out pastry onto a large sheet of parchment paper or a large pastry mat in a rectangle (up to 11"x17"). Using a paring knife, lightly score the pastry into three columns (I usually eyeball it - as you can tell from my pictures it's not 100% even, but it doesn't affect the taste!). You want the middle to be wider than the outer thirds. Cut even slits on each outer column - as many as you want. The more you have, the longer it's going to take you to wrap.
Pour your strudel filling down the center of the pastry and fold the center section of the pastry over at the top and bottom. Now start wrapping the center by pulling alternating slits of pastry from each side and pressing the ends into the pastry on the opposite side, pressing gently on the ends to connect to the base of the dough. Make sure you're not leaving big holes across the top!
Carefully transfer the pastry to your prepared baking sheet.
Brush the top of the folded pastry with an egg wash and then sprinkle raw sugar (or sparkling sugar) on top. You could also add some delicately placed almond slices on top if you wanted.
Bake for 25-30 minutes, or until top is golden brown.
Remove from oven, let cool on baking sheet for a few minutes, then transfer to a wire rack to cool before slicing into it.
Enjoy warm with a scoop of vanilla ice cream!