Melt butter in a medium-sized saucepan on medium heat.
Add apples, sugar, apple liqueur, lemon zest, raisins, and cinnamon.
Cook, stirring periodically for about ten minutes or until apples are soft but not falling apart.
Remove from heat and cool for ten minutes.
Strain out excess liquid in a sieve and reserve for making a glaze.
Stir walnuts into apple mixture.
Roll out puff pastry on a floured surface or a silicone mat to remove any seams from packaging.
Place apple mixture in a long line down the center of the pastry.
Fold one of the sides of the pastry lengthways over the apple filling and press to seal.
Repeat with the other half of the dough. Trim the top and bottom of any excess pastry.
Use a pastry brush to coat the top of the pastry with the beaten egg.
Use a knife to cut slits into the dough about every 1/2 inch, going almost all the way across the pastry.
Transfer to a parchment-lined cookie sheet.
Bake for 35 minutes or until the pastry is browned.
If you'd like to glaze the strudel, mix 5 teaspoons of reserved apple liquid with powdered sugar until you have a thick glaze. Drizzle over the strudel.
As a variation, try soaking the raisins in the liquor overnight before added them to the recipe. It makes them taste so good!This is best served the same day it's baked. If you have leftovers, store at room temperature in a container that is NOT airtight. That will help to keep the pastry crisp.If you are serving this a day or two after you make it, you can reheat it in the oven or toaster oven to crisp up the pastry again.Wrap tightly in plastic wrap and place in an airtight to container to freeze.