Preheat the oven to 375 degrees. Cut a circle around the top of the sourdough loaf and remove the inner bread to create a bowl shape. Set the removed bread pieces aside for dipping.
In a 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 cheese mixture. Mix until evenly distributed.
Spoon the dip into the hollowed bread bowl. Place the filled loaf on a baking sheet.
Bake for 20 to 25 minutes until the top is lightly golden and the dip is hot throughout. Serve with the reserved bread pieces.