Heat oven to 400'F. Beat the egg and 1 tbsp. water to make an egg wash. Set aside.
Stir the cheeses together with the onion and garlic powder in a small bowl.
Sprinkle the flour on your work surface and roll the pastry out the long way to about 12-14".
Brush the sheet with your egg wash and top with the cheese and the spinach. Starting at the short side roll up tightly like a jelly roll. Cut into slices about 1/2" wide.
Place the slices cut side down on a parchment covered baking sheet. Brush the slices with the egg wash and bake for 15 minutes, or until golden brown.
Remove the pastries onto a wire rack and cool for at least 10 minutes.
Make sure you wring out the thawed spinach well. I put it in a tea towel and twisted the ends to remove the water from the spinach. This works well if you have a friend to help twist the towel. If the spinach isn't dried out it will make the pastry soggy. Serve warm or cold.