Preheat oven to 400 degrees Fahrenheit (200 degrees Celsius) and line a baking sheet with parchment paper.
Place thawed spinach in a clean kitchen towel and squeeze forcefully to remove all excess moisture until the spinach is very dry.
In a mixing bowl, combine the dried spinach, feta, Parmesan, egg, oregano, garlic powder, pepper, and lemon zest.
Mix using a fork until the ingredients are evenly distributed and a thick paste forms.
Portion 1-tablespoon mounds of the mixture onto the prepared baking sheet, spaced 2 inches apart.
Flatten each mound using the back of a spoon or a piece of parchment paper until approximately 0.125 inches thick.
Bake for 12 to 15 minutes until the edges are golden brown and the centers are set.
Remove from oven and let sit on the baking sheet for 5 minutes to firm up before transferring to a wire rack.