Preheat your oven to 375°F (190°C).
Cut the loaf of bread in half lengthwise and hollow out the center of each half, leaving about a 1-inch shell.
In a mixing bowl, combine the cooked spaghetti, marinara sauce, shredded mozzarella cheese, and a pinch of salt and pepper. Mix until well combined.
Spoon the spaghetti mixture into the hollowed-out bread halves, packing it firmly.
In a small bowl, mix the melted butter with minced garlic and chopped parsley. Brush or drizzle this mixture over the top of the stuffed bread halves.
Sprinkle the grated Parmesan cheese evenly over the tops of the bread.
Place the stuffed bread halves on a baking sheet and bake for 20-25 minutes, or until the bread is crispy and golden and the cheese is melted and bubbly.
Let the stuffed bread cool for a few minutes before slicing and serving.