First make the filling. In a bowl, add together the cubed apples, blueberries, sugar, vanilla, cinnamon and lemon zest and juice. Toss everything together and allow to sit while you prepare the puff pastry.
Unfold puff pastry on a floured surface and roll out a bit so the pastry is a bit thinner.
Then cut pastry sheet into 9 even squares (18 total for both sheets) and add about a tablespoon of filling to each square.
Whisk together the egg and water in a small bowl and brush the edges of each square with the egg wash and then folding the dough together making a little pouch. Use your fingers or a fork to crimp the edges and it's ok if you stretch the dough a bit, the fruit will shrink as it bakes.
Brush more egg wash on top of the folded pastry and sprinkle with more sugar. Use a knife to cut little skits on the top.
Place borekas on lined baking sheet and bake at 325 for 20-25 minutes, or until the filling begins to ooz out and the pastry is golden brown.
Once done, remove from the oven and allow to cool a bit while you make tahini glaze.
To make the tahini glaze, in a bowl add the tahini and honey and 1/4 warm water. Use a fork or whisk to whisk everything together until it is smooth and pourable. As it sits, it will get firmer so you can add a bit more warm water and mix until desired consistency.