Preheat your oven to 375°F (190°C) and line a large baking sheet with heavy-duty aluminum foil. Place the round sourdough boule on a sturdy cutting board.

Using a long serrated bread knife, slice diagonally across the loaf in 1-inch intervals, cutting down close to the bottom crust but stopping about 1/2 inch before cutting all the way through. Rotate the loaf 90 degrees and slice diagonally again to create a neat 1-inch diamond grid pattern across the entire boule.

In a medium heatproof bowl, whisk together the melted European butter, minced garlic, finely chopped parsley, Italian seasoning, flaky sea salt, and optional red pepper flakes until fragrant and thoroughly combined.

Gently pry open the diamond cuts with your fingers. Using a silicone pastry brush or small spoon, drizzle about half of the garlic herb butter deep down into all the crevices and cuts.

Combine the shredded mozzarella and provolone in a bowl. Generously stuff the cheese blend deep into every single slit and pocket between the bread diamonds, packing it tightly so the loaf puffs with melted cheese.

Brush the remaining garlic herb butter generously over the entire top and sides of the loaf, then scatter the freshly grated Parmigiano-Reggiano evenly over the surface.

Wrap the loaf completely in the aluminum foil, creating a loose tent over the top so the cheese doesn't stick to the foil. Bake at 375°F (190°C) for 15 minutes until the cheese is completely melted deep inside.

Carefully open and peel back the foil to fully expose the top. Return to the oven and bake uncovered for another 8 to 10 minutes (or broil on high for 2 minutes) until the crust is deeply golden, crispy, and the cheese is bubbling with golden-brown toasted spots.

Remove from the oven, garnish with additional freshly chopped parsley, and let rest for 3 to 5 minutes so guests can pull individual cheesy wedges safely without scalding. Serve immediately with warm marinara dipping sauce!