Press the thawed spinach against a strainer with your hands until all of the water is out - you want to get as much out as possible so you don't have a soggy filling.
Mix the spinach, feta, scallions, olive oil, parsley, garlic and some salt and pepper - taste the mixture and adjust the seasonings as you please.
Beat one of the eggs in a small bowl and then fold it into the spinach mixture.
Roll out the defrosted puff pastry dough on a floured surface and cut it into 12 squares if you're planning on using mini muffin tins. I started by cutting my first sheet into 3 long strips and then into 4 smaller squares. Depending on how much filling you use in each puff, you might need to use the second sheet of puff pastry.
Place a square of dough into the muffin tin, push it against the sides and leave the corners draped over the edge - then spoon some of the spinach mixture into the center and fold the corners of the dough over the filling and pinch them together in the center.
Once you've used up all of the filling, beat the remaining egg with a tbsp of water and brush the tops of the puff pastry with the egg wash and bake at 400°F until the pastry is golden and puffed - approximately 20 minutes.
Run a sharp knife around the edges of the pan to loosen and turn them out onto a rack to cool a bit before serving.