Step 1: Preheat your oven to 375°F (190°C). Line a baking sheet with parchment paper or foil, or prepare a cast iron skillet.
Step 2: In a large bowl, combine the softened cream cheese, mayonnaise, sour cream, and minced garlic. Mix until smooth and well combined.
Step 3: Add the squeezed dry spinach, chopped artichoke hearts, 1 cup of shredded mozzarella cheese, 1/2 cup of grated Parmesan cheese, and black pepper to the cream cheese mixture. Stir thoroughly until all ingredients are evenly distributed.
Step 4: Place the sourdough boule on your prepared baking sheet or in the skillet. Using a sharp serrated knife, make deep crosshatch cuts into the top of the bread, about 3/4 of the way down, without cutting through the bottom crust. Space cuts about 1 inch apart in both directions.
Step 5: Gently open the cuts in the bread and spoon the spinach and artichoke filling into all the cracks and crevices. Be generous, making sure to get the mixture deep into the loaf. Some filling may sit on top, which is fine.
Step 6: Sprinkle the remaining 1/2 cup of mozzarella cheese and additional Parmesan cheese over the top of the bread.
Step 7: Loosely cover the bread with aluminum foil. Bake for 20 minutes.
Step 8: Remove the foil and continue baking for another 10-15 minutes, or until the cheese is melted and bubbly, and the top is golden brown. If the top is browning too quickly, you can loosely tent it again with foil.
Step 9: Remove from the oven and let it cool for 5-10 minutes before serving. This allows the cheese to set slightly, making it easier to pull apart. Serve warm and enjoy!