Heat olive oil in a skillet over medium heat.
In a bowl, mix the shredded chicken with marinara sauce, Italian seasoning, salt, and pepper.
Place one slice of sourdough bread in the skillet. Top with half of the mozzarella cheese, then add the chicken mixture, and sprinkle with Parmesan cheese.
Add the remaining mozzarella cheese on top, then place the second slice of bread over the top.
Cook for about 3-4 minutes until the bottom slice is golden brown. Carefully flip the sandwich and add butter to the skillet.
Cook for another 3-4 minutes until the other side is golden brown and the cheese is melted.
Remove from the skillet, let it cool for a minute, then slice in half and serve.