In a skillet, heat the olive oil over medium heat. Add the chopped spinach and sauté for 2-3 minutes until wilted.
In a bowl, combine the sautéed spinach, feta cheese, sun-dried tomatoes, black olives, garlic powder, salt, and pepper. Mix well.
Spread butter on one side of each slice of bread.
Place one slice of bread, butter side down, in a skillet over medium heat.
Spoon the spinach mixture onto the bread in the skillet, spreading it evenly.
Top with the second slice of bread, butter side up.
Cook for 3-4 minutes until the bottom slice is golden brown, then 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 and serve.