Prepare the Simple Syrup: In a small saucepan, combine the water and granulated sugar. Warm over medium heat, stirring until the sugar completely dissolves. Take off the heat and allow it to cool. Set aside to brush onto the croissants later.

Make the Apple Pie Filling: In another saucepan, combine the diced apples, brown sugar, cornstarch, ground cinnamon, water, and lemon juice. Cook over medium heat, stirring occasionally, until the apples are softened, and the mixture thickens (about 10 minutes). Remove from heat and let it cool.

Prepare the Almond Frangipane: In a mixing bowl, cream the softened butter, almond flour, powdered sugar, and salt together until smooth. Incorporate the eggs one by one, making sure to mix thoroughly after each addition. Add the almond extract (if using) and mix until the frangipane is creamy and well-blended.

Assemble the Croissants: Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper. Slice each croissant in half horizontally and brush the inside lightly with the prepared simple syrup. Spread a layer of almond frangipane on the bottom half of each croissant, followed by a spoonful of apple pie filling.

Top the Croissants: Spread a thin layer of frangipane over the top of each assembled croissant. Sprinkle sliced almonds on top to cover the frangipane layer.

Bake: Place the croissants on the prepared baking sheet and bake for 15-20 minutes, or until the tops are golden brown and the frangipane is set.

Serve: Allow the croissants to cool slightly, then dust with powdered sugar before serving.