Preheat your oven to 375°F (190°C).
Place your loaf of sourdough bread on a cutting board. Using a sharp knife, make vertical cuts about 1 inch apart across the top of the loaf, being careful not to slice all the way through. Stop about ½ inch from the bottom crust to keep the loaf intact. Turn the loaf 90 degrees and make another set of 1-inch spaced cuts to create a crisscross pattern. You’ll end up with square-ish pieces perfect for stuffing.
In a small bowl, stir together the melted butter, garlic powder, and salt. Set aside.
Shred the mozzarella and Parmesan cheese (freshly shredded melts better than pre-shredded!). Toss them together to evenly combine.
Use a pastry brush or spoon to drizzle the garlic butter mixture into all the crevices of the bread. Gently pull the dough pieces apart and stuff with the shredded cheese blend. Sprinkle chopped parsley (or your favorite fresh herbs) over the top.
Place the stuffed loaf on a piece of parchment paper. Then wrap it completely in aluminum foil, but loosely. Form a little tent over the top so the foil doesn’t stick to the cheese. You want it fully enclosed but not pressed tightly against the bread.
Transfer the foil-wrapped loaf to a baking sheet and bake for 30 minutes. Then carefully unwrap the foil and bake uncovered for another 5–10 minutes, until the top is golden brown and the cheese is melty. Serve warm and enjoy!