Preheat & Prep Baking Pan: Preheat your oven to 375°F (190°C). Line a large rimmed baking sheet with a generous double layer of heavy-duty aluminum foil, leaving enough overhang to wrap the loaf.

Slice Diamond Crosshatch Pattern: Using a sharp serrated bread knife, slice the sourdough boule diagonally at 1-inch intervals, cutting down about 1 inch from the bottom crust (do NOT cut completely through the bottom base). Turn the loaf 90 degrees and slice diagonally again in 1-inch intervals to create an even diamond grid pattern.

Whisk Garlic Herb Butter: In a small bowl, whisk together the melted European butter, minced garlic, finely chopped parsley, Italian seasoning, and kosher salt. Use a pastry brush to generously brush the butter into all deep cuts and over the top crust.

Mix Spinach Artichoke Filling: In a medium mixing bowl, beat softened cream cheese, sour cream, and mayonnaise until smooth. Fold in thoroughly dried chopped spinach, chopped artichoke hearts, grated parmesan, 1 cup of mozzarella cheese, red pepper flakes, and black pepper until completely combined.

Stuff the Loaf: Using a spoon or clean hands, generously stuff the spinach artichoke mixture deep into every crevice and between all diamond cuts of the bread. Push remaining 1 cup of mozzarella into cracks and scatter across the top surface.

Foil Wrap & Bake: Bring the foil overhang up and wrap the loaf loosely so the foil does not touch the melting cheese directly. Bake wrapped for 20 minutes until the filling is steaming hot and melted.

Unwrap & Broil to Golden Perfection: Carefully fold back the foil to expose the top of the bread. Return to the oven and bake uncovered for 8 to 10 minutes (or broil on high for 2 minutes) until the mozzarella is golden-brown, bubbling furiously, and the bread edges are deeply toasted and crunchy.

Garnish & Serve: Remove from oven, let rest for 3 to 5 minutes, garnish with freshly minced parsley and cracked black pepper, and serve warm directly on a board for guests to pull apart.