Preheat the oven to 375°F. Line a baking sheet with foil.
Cut a circle around the top of the sourdough loaf and remove the lid. Hollow out the center, leaving a 1-inch thick wall on all sides.
In a large mixing bowl, combine the cream cheese, sour cream, Parmesan, mozzarella, minced garlic, salt, and black pepper. Stir until smooth.
Add the drained spinach and chopped artichoke hearts to the bowl. Mix until evenly distributed.
Spoon the mixture into the hollowed sourdough bowl. Place the filled loaf on the prepared baking sheet.
Bake for 25 minutes, or until the top is lightly browned and the dip is hot throughout. Serve the bread lid and removed bread chunks on the side for dipping.