Preheat Oven & Prep Foil: Preheat your oven to 375°F (190°C). Tear off two large sheets of heavy-duty aluminum foil and place them overlapping on a rimmed baking sheet.

Cross-Hatch the Sourdough: Using a sharp serrated bread knife, slice the sourdough boule into 1-inch thick slices, cutting down to within 1/2-inch of the bottom crust without cutting all the way through. Rotate loaf 90 degrees and slice again to create a grid of bread "fingers." Place the sliced loaf in the center of the foil.

Whisk Garlic Herb Butter: In a small bowl, whisk together melted butter, olive oil, grated garlic, chopped parsley, minced rosemary, fresh thyme, salt, and red pepper flakes until fragrant.

Stuff the Bread: Combine shredded mozzarella and white cheddar. Using your fingers, gently pry open each crevice of the bread grid. First, spoon about half of the garlic herb butter deep into all the cracks. Then, generously stuff the shredded cheese mixture deep into every crevice. Drizzle the remaining garlic butter over the entire top of the loaf, and sprinkle grated parmesan all over.

Wrap & Bake: Loosely wrap the foil up around the sourdough boule to seal in the steam and melt the cheese without burning the crust. Bake covered for 15 minutes.

Unwrap & Crisp to Golden: Carefully open the foil, exposing the top of the bread. Return to the oven and bake uncovered for 8 to 10 minutes until the top cheese is bubbling, melted, and deeply golden brown at the crust edges.

Garnish & Serve Hot: Sprinkle fresh chopped parsley over the top. Serve immediately on a wooden board while piping hot, pulling cheesy pieces directly from the loaf alongside warm marinara sauce for dipping!