Preheat oven to 375°F (190°C).
Using a serrated knife, cut the sourdough loaf into a grid pattern, making sure not to slice all the way through. Leave the base intact.
In a large bowl, combine the cream cheese, sour cream, mayonnaise, garlic, spinach, artichoke hearts, mozzarella, and half of the Parmesan. Mix until fully incorporated. Season with salt and pepper.
Carefully stuff the cheesy mixture into the cuts of the bread, pushing it deep into the crevices.
Drizzle melted butter over the top and sprinkle with the remaining Parmesan.
Wrap the bread loosely in foil and bake for 20 minutes.
Uncover and bake an additional 10-15 minutes, until the top is golden and bubbly.
Garnish with chopped parsley and serve warm.
