In a bowl, combine egg and water with fork. Set aside.
With the rack in the middle position, preheat the oven to 200 °C (400 °F). Line a baking sheet with parchment paper.
Press apples with your hands to remove excess juice.
In a bowl, combine apples with lemon juice. Add maple butter. Set aside.
On a lightly floured work surface, roll out dough into a 25-cm (10-inch) square. Cut into four squares. Place apple mixture in the centre of each square. Brush two perpendicular sides of each square with egg wash. Close them to form triangles. With a fork, press edges to seal dough.
Place turnovers on the baking sheet. With scissors, make small cuts in dough in the centre of each turnover to allow steam to escape. Brush with remaining egg wash and sprinkle with sugar, if desired. Bake for about 25 minutes or until turnovers are golden brown. Let cool.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.