Heat a skillet over medium heat and add the olive oil.
In a small bowl, mix the crumbled feta, cream cheese, garlic powder, salt, and pepper until well combined.
Spread the cheese mixture evenly on one side of each slice of bread.
On one slice, layer the fresh spinach on top of the cheese mixture.
Place the other slice of bread on top, cheese side down, to form a sandwich.
Melt the butter in the heated skillet.
Carefully place the sandwich in the skillet and cook for about 3-4 minutes, or until the bread is golden brown.
Flip the sandwich and cook for another 3-4 minutes on the other side until golden and the cheese is melted.
Remove the sandwich from the skillet, let it cool for a minute, then slice in half and serve warm.