Use a sharp, serrated knife to cut long rows in the loaf of bread, about 1 inch apart, but do NOT cut through the bottom of the loaf. Turn the loaf 90 degrees and cut more rows, about 1 inch apart, but not cutting through to the bottom.
Combine the melted butter and pesto in a small dish and carefully open the cracks of the bread to drizzle the sauce in. If needed, use a pastry brush to get in between the cracks.
Carefully stuff each crack with the mozzarella cheese, evenly around the entire loaf. Place the loaf on a sheet of tinfoil and fold up sides, place another sheet of foil on top and cover completely. Place on a baking sheet and bake for 20 to 25 minutes, or until cheese in center is melted. Uncover the loaf and bake for an additional 5 minutes. Garnish with fresh parsley and enjoy!