Preheat oven to 375F°. Prepare a baking sheet with a silicone baking mat or parchment paper.
Carefully open up a thawed sheet of puff pastry on your prepared baking sheet. One giant strudel can be made or several small ones. I sliced my sheet into two pieces. You can go even smaller, just remember to divide your filling ingredients accordingly.
Spread blueberries down the center of your puff pastry. A nice neat line, no more than a few blueberries wide.
In a medium bowl toss apples, cinnamon, nutmeg and brown sugar and butter until the apples are coated well. Top off blueberries with apple mixture. In a small bowl, beat egg with water. Brush edges of puff pastry with egg wash.
Fold sides of puff pastry in like you would with a burrito. Then pull the remaining puff pastry up to meet in the middle over your line of fruit filling. Pinch well to seal. Roll strudel over carefully so the seam will be on the bottom. Brush with remaining egg wash over entire strudel.
Sprinkle with turbinado sugar.
Slice the top several times. Refrigerate for 20 minutes. Then bake at 375F° for 25-30 minutes. Cool on the cookie sheet for 5-10 minutes to harden up a bit, then carefully transfer to a wire rack to cool completely. Slice and serve.