01 - Preheat oven to 350°F and adjust rack to center position.
02 - Using a serrated bread knife, cut a circular opening in the top of the sourdough loaf and remove the lid. Scoop out the soft interior, leaving approximately 3/4 inch of bread around the sides and bottom to create a sturdy bowl. Set aside the removed bread for cubing.
03 - In a large mixing bowl, combine softened cream cheese, sour cream, mayonnaise, shredded mozzarella, grated Parmesan, minced garlic, diced onion, salt, black pepper, and nutmeg. Mix thoroughly until completely smooth and uniform.
04 - Add the well-drained spinach to the cheese mixture, breaking up any clumps with a fork. Stir until spinach is evenly distributed throughout the dip base.
05 - Transfer the spinach dip mixture into the hollowed bread bowl, packing it gently. 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. Bake for 20 minutes to heat through and melt the cheeses completely.
07 - Remove the aluminum foil and continue baking for an additional 5 minutes, or until the top is golden brown and bubbly.
08 - While the dip bakes, cut the reserved sourdough bread into 1-inch cubes for serving.
09 - Present the warm spinach dip directly in the bread bowl. Arrange the toasted bread cubes and fresh vegetable crudités around the bowl for dipping.