01 - Preheat oven to 350°F.
02 - Cut a circular opening in the top of the sourdough loaf and remove the lid. Hollow out the center, leaving about 0.75 inch of bread around the sides and bottom to form a sturdy bowl. Reserve all removed bread pieces for dipping.
03 - In a large mixing bowl, combine cream cheese, sour cream, mayonnaise, mozzarella, Parmesan, garlic, onion, salt, pepper, and nutmeg. Mix until completely smooth and well incorporated.
04 - Add the thoroughly drained spinach to the cheese mixture. Stir until evenly distributed throughout the dip.
05 - Spoon the spinach dip mixture into the prepared bread bowl. Place the filled bread bowl on a baking sheet lined with parchment paper.
06 - Cover loosely with aluminum foil and bake for 20 minutes to heat through.
07 - Remove foil and bake for an additional 5 minutes, or until the top becomes golden brown and bubbly.
08 - While dip bakes, cut reserved bread pieces into bite-sized cubes for serving.
09 - Serve the warm spinach dip directly in the bread bowl, surrounded by bread cubes and assorted fresh vegetables for dipping.