In a bowl, combine the chopped spinach, feta cheese, sun-dried tomatoes, kalamata olives, olive oil, garlic powder, oregano, salt, and pepper. Mix well to create a flavorful filling.
Take one slice of bread and spread half of the spinach mixture evenly on top. Sprinkle half of the shredded mozzarella cheese over the filling, then top with the second slice of bread.
Heat a skillet over medium heat. Add a drizzle of olive oil to the skillet. Once hot, place the sandwich in the skillet and cook for about 3-4 minutes on one side until golden brown and crispy.
Carefully flip the sandwich and add the remaining spinach mixture and mozzarella cheese on the top slice. Cook for another 3-4 minutes, pressing gently to ensure even cooking, until the cheese is melted and the bread is golden brown.
Remove from the skillet, allow to cool for a minute, then slice in half and enjoy while warm!