Mix water, cinnamon, and sugar in a measuring cup and pour over apples. Mash with a potato masher or place in a blender to blend with no lumps. Use a knife to cut off the rounded edges, to give you an exact rectangle. Reserve the edges you cut off. Layout your crust rectangles on a flat surface. Whisk egg in a small bowl. Use your finger and trace the edges of each of the rectangles containing applesauce with a bit of egg, to help in sealing the pop tart. Cover the applesauce rectangles with a plain one and seal the edges together with the tines of a fork. Place pop tarts on a cookie sheet lined with parchment. Poke each pop tart with a fork a few times so that air can escape while baking, so they don't expand.