Preheat your oven to 375°F (190°C).
In a large mixing bowl, combine the cooked spaghetti, marinara sauce, and half of the mozzarella cheese. Mix well until the spaghetti is evenly coated.
In another bowl, mix the softened butter, minced garlic, chopped parsley, Italian seasoning, salt, and pepper to create a garlic butter spread.
Slice the French bread in half lengthwise, creating a top half and a bottom half.
Spread the garlic butter mixture evenly on both halves of the bread.
Spoon the spaghetti mixture onto the bottom half of the bread, spreading it evenly.
Top the spaghetti with the remaining mozzarella cheese.
Place the top half of the bread back on and wrap the entire loaf in aluminum foil.
Bake in the preheated oven for 20 minutes.
Remove the foil and bake for an additional 5 minutes, or until the cheese is melted and the bread is golden brown.
Remove from the oven, slice into individual pieces, and serve warm.