Heat a skillet over medium heat.
In a bowl, combine the chopped spinach, feta cheese, mozzarella cheese, garlic powder, black pepper, and salt. Mix well.
Spread 1 tablespoon of softened butter on one side of each slice of bread.
Place one slice of bread, buttered side down, in the skillet.
Spoon the spinach and cheese mixture onto the bread in the skillet.
Top with the second slice of bread, buttered side up.
Cook for about 3-4 minutes, or until the bottom slice is golden brown.
Carefully flip the sandwich and cook for another 3-4 minutes until the other side is golden and the cheese is melted.
Remove from the skillet, let cool for a minute, then slice in half and serve.