In a mixing bowl, combine the ricotta cheese, chopped sun-dried tomatoes, garlic powder, salt, and pepper. Stir until well mixed.
Heat a non-stick skillet over medium heat.
Spread butter on one side of each slice of sourdough bread.
Lay two slices of bread in the skillet, buttered side down.
On each slice of bread, spread a generous amount of the ricotta and sun-dried tomato mixture.
Top the mixture with the chopped spinach and then sprinkle shredded mozzarella cheese on top.
Place the remaining two slices of bread on top of the cheese, buttered side up.
Cook for about 3-4 minutes or until the bottom slice is golden brown. Carefully flip the sandwiches and cook the other side for another 3-4 minutes, until golden brown and the cheese has melted.
Once done, remove from the skillet and let the sandwiches cool for a minute before slicing in half.