Prep & Preheat: Preheat oven to 375°F (190°C). Line a large rimmed baking sheet with aluminum foil and parchment paper on top.

Slice Bread Grid: Using a sharp serrated bread knife, slice the sourdough boule into a 1-inch diamond grid crosshatch pattern, cutting down within 1/2 inch of the bottom crust without cutting all the way through.

Mix Garlic Herb Butter: In a small bowl, whisk together melted butter, minced garlic, chopped parsley, and salt. Use a pastry brush to brush half of the garlic butter deep inside all the sliced bread crevices.

Mix Spinach Artichoke Filling: In a medium bowl, stir together softened cream cheese, sour cream, squeezed dry spinach, chopped artichoke hearts, 1 cup of mozzarella, 1/4 cup of parmesan, and red pepper flakes until thick and creamy.

Stuff the Loaf: Using clean fingers or a small spoon, pack the spinach artichoke filling generously into all the crevices between the bread cubes. Stuff the remaining 1 cup of shredded mozzarella and 1/4 cup parmesan cheese into the cuts and over the top. Brush the remaining garlic herb butter over the crust.

Bake to Molten Perfection: Wrap the loaf loosely in foil and bake at 375°F (190°C) for 15 minutes to melt the interior cheese. Unwrap foil and bake uncovered for 8 to 10 minutes more until the top is bubbling, golden-brown, and crispy. Garnish with chopped parsley and serve hot immediately.