Thaw spinach completely in a colander. Once thawed, using your hands, tightly squeeze all the water out of the spinach handful by handful. Keep breaking up the spinach to move it around and ensure that all the water is completely out.
Thaw the puff pastry at room temp for about 40 minutes or until easy to handle.
Heat oven to 400 degrees. Lightly grease a baking sheet or line with parchment paper.
Grate 3/4 cup cheddar and 3/4 cup parmesan cheese. Add garlic and onion powder to the cheese along with the freshly grated black pepper.
Lightly beat the egg with 1 Tbsp. water.
Unfold the thawed pastry onto a lightly floured surface, lightly flour the top, and gently roll out to about 1/4 inch thickness. Brush with the egg wash.
Next top with one of the cheeses and 1/4 cup of the scallions.
Sprinkle on the spinach and the last 3/4 cup of cheese.
Starting at the long side, roll the pastry into a fairly tight roll, using your fingers to push the filling back in as you roll.
Cut into 1/2 inch pieces and place on the prepared baking sheet. Brush liberally with the last of the egg wash (use it all) and place in the fridge for 10 minutes.
Bake for 30 minutes (check at 25 to make sure it's not getting too browned) and once golden remove from the oven to cool.
Serve either warm right from the oven or also delicious at room temperature.