In a medium-sized mixing bowl, stir together all filling ingredients until combined and set aside.
Cook shells according to package instructions. Drain, then spread them out on a baking sheet or cutting board to prevent them from sticking together.
Preheat oven to 350°F (177ºC) and spread marinara across the bottom of a 9x13-inch baking pan. Stuff shells with equal amounts filling and place each filled shell into the pan on top of the sauce as you go.
Sprinkle remaining mozzarella across the top, if using, and bake for 30 minutes. Optional: Once baked, place shells under broiler for 2 minutes keeping a close eye until the cheese is slightly browned and crispy. Garnish with fresh parsley, transfer to serving plates and enjoy!