Preheat your oven to 375°F (190°C).
In a medium saucepan, combine the cooked spaghetti with marinara sauce over medium heat. Stir until evenly coated and heated through. Remove from heat and set aside.
Take the large loaf of Italian bread and slice it in half lengthwise. Use a knife to hollow out some of the bread from each half, creating a well for the filling.
In a mixing bowl, blend the softened butter with minced garlic, dried Italian herbs, salt, and pepper.
Spread the garlic butter mixture generously over the inside of each bread half.
Fill each half of the bread with the cooked spaghetti mixture. Ensure to pack it well.
Top the filled bread halves with shredded mozzarella cheese.
Wrap each half in aluminum foil and place them on a baking sheet.
Bake in the preheated oven for 15 minutes. After 15 minutes, remove the foil and bake for an additional 5 minutes or until the cheese is melted and slightly golden.
Remove from the oven and let it cool for a few minutes. Garnish with chopped parsley before slicing into pieces to serve.