Preheat the oven to 425°. Line a baking sheet with parchment paper. On a lightly floured work surface, using a lightly floured rolling pin, roll out the puff pastry 1/8 inch thick. Using a 3 1/2-inch round biscuit cutter, cut out 12 rounds; transfer them to the prepared baking sheet. Working with one round at a time, brush the edge with some of the beaten egg, then mound about 1 tablespoon of the spinach filling in the center. Bring three sides of the dough together in the center to form a triangle and pinch to seal. Repeat with the remaining pastry rounds, egg wash and spinach filling.