01 - Preheat the 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 the removed bread for dipping.
03 - In a large mixing bowl, combine the cream cheese, sour cream, mayonnaise, mozzarella, Parmesan, garlic, onion, salt, pepper, and nutmeg. Mix until completely smooth and well incorporated.
04 - Add the well-drained spinach to the mixture and stir until evenly distributed throughout the dip.
05 - Spoon the dip mixture into the prepared bread bowl. Place the filled bread bowl on a baking tray lined with parchment paper.
06 - Cover loosely with aluminum foil and bake for 20 minutes.
07 - Remove the foil and bake for an additional 5 minutes, or until the top is golden brown and bubbly.
08 - While the dip bakes, cut the reserved bread into bite-sized cubes for serving.
09 - Serve the warm spinach dip in the bread bowl, surrounded by bread cubes and assorted vegetables for dipping.