Spread 1 tbsp of butter to the edges of a slice of sourdough bread. Set it face down in a medium skillet.
Spread 1 tbsp of pesto on the side of the bread without butter, spreading it as evenly as possible.
Top the pesto side with half the mozzarella cheese, then sliced turkey, then chopped spinach. Add the remaining mozzarella cheese on top of the spinach.
Spread the remaining pesto on one side of the second slice of bread and place it face down onto the sandwich in the skillet, so the mozzarella cheese and the pesto are touching.
Spread the remaining tbsp of butter on the dry side of the second slice of bread. Heat your turkey sandwich on medium-low until brown and golden, about 2-3 minutes.
Flip the sandwich, and brown the other side for about 2-3 minutes. Cut in half, and split with a friend!