Heat the olive oil in a skillet over medium heat. Add the chopped spinach and sauté for about 2-3 minutes until wilted. Season with garlic powder, salt, and pepper. Remove from heat and set aside.
In a separate bowl, mix the crumbled feta cheese with the sautéed spinach. If you like a bit of heat, add the red pepper flakes.
Spread butter on one side of each slice of bread. On the unbuttered side of one slice, spread the spinach and feta mixture evenly. Top with the other slice of bread, buttered side up.
Place the sandwich in the skillet over medium heat. Cook for about 3-4 minutes until the bottom 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 it cool for a minute, then slice in half and serve warm.