Thaw the puff pastry at room temperature for 15 minutes before rolling, and mix the spinach filling until smooth. Preheat your oven to 400°F (205°C) and line a baking sheet with parchment.
Roll the thawed puff pastry into a 10×12-inch rectangle on a lightly floured surface, smoothing out any creases.
Mix the squeezed-dry spinach, crumbled feta, softened cream cheese, garlic powder, and black pepper in a bowl until evenly combined and creamy.
Spread the spinach mixture evenly over the pastry, leaving a ½-inch border along one long edge for sealing, then roll tightly from the opposite long edge into a log.
Wrap the log in plastic wrap and chill in the fridge for 20 minutes, until firm enough to slice cleanly.
Slice the chilled log into ½-inch rounds, arrange on the prepared baking sheet, brush tops with egg wash, and bake for 18-20 minutes until puffed and golden-brown.
