Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
Roll out the thawed puff pastry on a lightly floured surface to smooth out any creases.
Using a sharp knife or pizza cutter, cut the puff pastry into squares, approximately 3x3 inches each. You should have about 12-16 squares, depending on how thick you roll the pastry.
Place about a tablespoon of cranberry sauce in the center of each puff pastry square.
Add a small piece of Brie cheese on top of the cranberry sauce. If using, sprinkle a few chopped walnuts or pecans over the cheese.
Fold the corners of each pastry square towards the center, pinching to seal them together, creating a little pouch.
Brush the tops of the pastry bites with the beaten egg to achieve a golden color as they bake.
Place the pastries on the prepared baking sheet, evenly spaced apart.
Bake in the preheated oven for 15-20 minutes, or until they are golden brown and puffed up.
Once out of the oven, let them cool slightly before garnishing with fresh rosemary or thyme. Season with salt and pepper to taste, if desired.
Notes
Feel free to use homemade cranberry sauce for a personal touch.