Preheat the oven to 425F (220C). Line a baking sheet with parchment paper.
In a small bowl, combine the sugar and ground cinnamon.
1 tbsp ground cinnamon, 1/4 cup granulated sugar
Brush the puff pastry sheet with milk and sprinkle 1/2 of the cinnamon sugar mixture onto the pastry in an even layer. Cut the pastry into long even triangles.
1 puff pastry sheet, 1 tbsp milk
Peel, core, and cut the apples into small wedges. Roll them in the sugar and cinnamon mixture.
1 apple
Place a coated apple piece on the longer end of a triangle and tightly roll it up into a croissant. Lightly press in the end of the triangle to secure the shape. Don't overfill the croissants or they can open up during baking. Repeat with the remaining pastry triangles.
Brush the croissants with milk and roll them in the cinnamon mixture. Place them onto the lined baking sheet and bake for 10-15 minutes until puffed up and golden brown.
Let them cool slightly and serve warm or cold.
Notes
Don't overwork the dough or it can toughen the croissants.Cut the pastry sheet in half before cutting the triangles for even smaller apple croissants. You'll need to adjust the apple pieces accordingly.Storage: These croissants are best enjoyed fresh but they can be stored in an airtight container at room temperature for up to 2 days.