In a large mixing bowl, whisk together all-purpose flour and sea salt. Grate in cold salted butter using a box grater, then mix until the mixture resembles coarse crumbs. Pour in ice-cold water, pure maple syrup, and sourdough discard, combining until a shaggy dough forms. Be careful not to over-mix; some flour bits are perfectly fine.

On a lightly floured surface, roll the dough into a rectangle, roughly 12x8 inches. Fold the dough into thirds like a letter, ensuring a smooth and even fold. Wrap in plastic wrap and place it in the freezer for about 30 minutes.

Retrieve the chilled dough from the freezer and roll it out again into a rectangle, similar in size to the previous step. Fold the dough into thirds once more, then wrap it in plastic again. Return the folded dough to the freezer for another 30 minutes.

In a skillet over medium heat, melt salted butter, then add diced apples along with ground cinnamon, pure maple syrup, and lemon juice. Cook for about 5-7 minutes until the apples soften. Mix cornstarch with water to create a slurry, stir it in, and cook for an additional minute before removing from heat. Let the filling cool.

Preheat your oven to 425°F (220°C) and line a baking sheet with parchment paper. Roll out the dough to about 1/8-inch thickness and cut it into 4-inch squares. Spoon a generous amount of the cooled apple filling into the center of each square, fold the corners over to form a pocket, and seal edges with an egg wash.

Place the shaped turnovers on the prepared baking sheet, leaving space between each. Poke holes in the tops with a knife or fork to allow steam to escape while baking. Chill the assembled turnovers in the refrigerator for 10-15 minutes before brushing them with egg wash.

Bake the turnovers in the preheated oven for 20-24 minutes until golden-brown. Once out of the oven, allow the turnovers to cool slightly on a wire rack before drizzling with optional cinnamon frosting.