Preheat oven to 375°F (190°C). Line a large baking sheet with foil and parchment paper. Using a sharp serrated knife, slice the sourdough in a 1-inch crosshatch diamond grid, cutting deep but stopping 1/2-inch above the bottom crust so the base stays intact.
In a small bowl, whisk melted butter, minced garlic, parsley, salt, and pepper. Generously brush the garlic butter deep between all crevices of the loaf, reserving 1 tablespoon for the top.
In a mixing bowl, beat softened cream cheese and sour cream until silky. Fold in squeezed dry spinach, chopped artichoke hearts, 1 cup mozzarella, Parmesan, garlic powder, and red pepper flakes.
Using a small spatula or butter knife, gently pack the spinach artichoke mixture deep into every crevice of the sliced bread.
Top with the remaining 1/2 cup mozzarella and brush the top crust with reserved garlic butter. Tent loosely with foil and bake at 375°F for 20 minutes until heated through and bubbling.
Unfold the foil to expose the top and bake uncovered for 8 to 10 minutes until the cheese is browned and the crust is crackling golden.
Rest 3 minutes, garnish with fresh parsley and extra Parmesan, and serve warm for guests to pull apart and enjoy!