Heat the oven to 375°F.
Stir the melted butter, garlic, parsley, onion powder, Italian seasoning, salt, and black pepper together.
Cut the loaf in a crisscross pattern, stopping before the knife reaches the bottom crust so the loaf stays intact.
Brush the garlic-butter mixture deeply into the cuts, then stuff the shredded mozzarella and cheddar cheese evenly between the bread sections.
Wrap the loaf loosely in foil, place it on a baking sheet, and bake for 15 to 20 minutes.
Open the foil and bake for 10 to 15 minutes more, until the cheese is melted and the top is crisp.
Cool for a few minutes, sprinkle with the optional grated Parmesan if using, and serve warm.