Peel, core, and dice apples into small pieces.

In a medium saucepan, combine diced apples, brown sugar, cinnamon, lemon juice, cornstarch, and water. Stir until cornstarch is dissolved.

Cook over medium heat, stirring frequently, until apples are tender and sauce has thickened (about 5-7 minutes). Remove from heat and let cool slightly.

Preheat oven to 375°F (190°C). Line a baking sheet with parchment paper.

Unroll each croissant. Spoon 2-3 tablespoons of apple filling onto the wide end of each croissant.

Roll croissants from the wide end to the pointed end. Place seam-side down on the baking sheet.

Optional: Whisk egg with 1 tablespoon of water and brush over croissants.

Bake for 15-20 minutes, or until golden brown and puffed.

Let cool on baking sheet for a few minutes, then transfer to a wire rack to cool completely. Dust with powdered sugar if desired.