Preheat the oven to 180°C (350°F). Line a baking tray with parchment paper.
Using a sharp knife, cut the bread in a crosshatch pattern, slicing down but not all the way through so the loaf stays intact.
In a small bowl, mix melted butter, minced garlic, salt, and black pepper.
Gently pull open the cuts in the bread and stuff shredded mozzarella and cheddar deep into the gaps.
Brush the garlic butter generously over the entire loaf, letting it drip into the cuts.
Wrap the bread loosely in foil and bake for 15 minutes.
Uncover and bake for another 8–10 minutes until the top is lightly golden and the cheese is fully melted.
Sprinkle with parsley or parmesan if using. Serve warm.