Preheat oven to 350 degrees.
Place Italian sausage, egg, breadcrumbs, milk, and fresh parsley in a large mixing bowl and work mixture together until all the ingredients are well combined.
Use a cookie scoop to make evenly sized meatballs. Roll each meatball in the palm of your hands and place them on a cookie sheet.
Bake for 20 minutes or until they reach an internal temperature of 165℉.
While meatballs are baking, heat marinara sauce in a saucepan over medium heat.
Cut hoagie rolls in half horizontally and place them on a cookie sheet, cut side up.
Once meatballs are cooked through, remove them from the oven and set aside.
Place hoagie rolls in the oven and set temperature to "broil". Cook until rolls are toasted and golden brown.
Top each toasted hoagie roll with marinara sauce, meatballs and sprinkle with mozzarella cheese.
Return sandwiches to the oven and heat them until the cheese is melted.
Top with a sprinkle of Parmesan cheese if desired to serve.