Preheat your oven to 375°F. Line a baking sheet with parchment paper.
Carefully slice each croissant in half lengthwise, creating a pocket for the filling.
In a bowl, combine the shredded mozzarella cheese, pepperoni slices, and Italian seasoning. Mix well.
Spoon the cheese and pepperoni mixture evenly into each croissant half.
Close the croissants and place them on the prepared baking sheet.
Brush the tops of the croissants with the beaten egg to give them a golden finish.
Bake in the preheated oven for 12-15 minutes, or until the croissants are golden brown and the cheese is melted.
Remove from the oven and let cool for a few minutes before serving with marinara sauce for dipping.