Preheat your oven to 375°F (190°C).
In a mixing bowl, combine the cooked spaghetti with marinara sauce, mozzarella cheese, and Parmesan cheese. Mix well until the spaghetti is evenly coated.
Slice the French bread in half lengthwise. Carefully hollow out the inside of each half to create space for the spaghetti filling, leaving about half an inch of bread on the crust.
In a separate bowl, mix the softened butter, minced garlic, dried parsley, salt, and black pepper.
Spread the garlic butter mixture generously over the inside of both halves of the hollowed bread.
Spoon the spaghetti mixture into both halves of the bread, packing it tightly.
Place the stuffed bread halves on a baking sheet. If desired, wrap them in aluminum foil to keep them moist while baking.
Bake in the preheated oven for about 15-20 minutes, or until the cheese is melted and bubbly, and the bread is golden brown.
Remove from the oven and let cool for a few minutes before slicing into individual servings.