01 - Preheat your oven to 350°F for optimal baking conditions.
02 - Using a serrated bread knife, cut a circular opening in the top of the sourdough loaf and remove the lid. Carefully hollow out the center, maintaining approximately ¾ inch of bread around the sides and bottom to create a sturdy bowl. Set aside the removed bread pieces for dipping.
03 - In a large mixing bowl, combine the softened cream cheese, sour cream, mayonnaise, shredded mozzarella, grated Parmesan, minced garlic, chopped onion, salt, black pepper, and nutmeg. Beat with a spatula or electric mixer until completely smooth and well incorporated.
04 - Add the thoroughly drained spinach to the cheese mixture. Fold gently until the spinach is evenly distributed throughout the dip base.
05 - Transfer the spinach dip mixture into the prepared bread bowl, filling it generously. Place the filled bread bowl on a baking sheet lined with parchment paper for easy cleanup.
06 - Cover the bread bowl loosely with aluminum foil to prevent excessive browning. Bake for 20 minutes to heat through and melt the cheeses.
07 - Remove the foil and continue baking for an additional 5 minutes, or until the top becomes golden brown and bubbly.
08 - While the dip bakes, cut the reserved bread pieces into uniform 1-inch cubes for serving.
09 - Present the warm spinach dip directly in the bread bowl, surrounded by bread cubes and assorted fresh vegetables for dipping.