Place your sourdough or boule loaf on a cutting board. Use a serrated knife to make a crosshatch pattern on top, slicing about 1 inch apart without cutting all the way through.

In a saucepan, melt 1/2 cup of unsalted butter over low heat. Stir in 4 minced garlic cloves and 2 tablespoons of chopped parsley, then let cool.

Brush the garlic butter mixture between the cuts in the loaf. Stuff the loaf with 2 cups of mozzarella and optionally 1/2 cup of Parmesan cheese.

Preheat oven to 350°F (175°C). Wrap the loaf tightly in aluminum foil and bake for 20 minutes.

Unwrap the foil and return the loaf to the oven for an additional 5 minutes to crisp the crust.

Remove the bread from the oven, let cool for a minute, and serve warm for guests to pull apart.