In a colander squeeze the excess water out of the spinach.
In a medium bowl combine the spinach and cream cheese together with a fork.
Stir in the white cheddar cheese, onion, garlic powder, salt and pepper.
In a small bowl beat the egg with 1 tablespoon water.
On a floured work surface unfold the puff pastry and brush it with the egg.
Spread the spinach mixture evenly from edge to edge. Roll it up.
Place it on a piece of parchment paper on a baking sheet, cover with plastic wrap and refrigerate for 20 minutes.
Preheat the oven to 400 degrees F.
Place it back on the floured work surface and brush all over with the egg wash.
With a serrated knife slice into ½-inch thick slices. Place on the parchment-lined baking sheet and inch apart.
Bake 15 minutes or until lightly golden around the edges and cooked through in the center.
Sprinkle with Parmesan cheese to serve.