01 - Preheat the oven to 350°F.
02 - In a mixing bowl, blend the cream cheese, sour cream, and mayonnaise until smooth and creamy.
03 - Fold in the thawed spinach, Parmesan, mozzarella, garlic, onion, salt, pepper, and nutmeg until evenly distributed.
04 - Slice off the top of the sourdough loaf and hollow out the interior, leaving approximately 3/4 inch thick walls. Reserve the removed bread for dipping.
05 - Spoon the spinach mixture into the prepared bread bowl and place it on a baking sheet.
06 - Bake uncovered for 30 minutes until the dip is bubbly and golden on top.
07 - During the last 10 minutes of baking, arrange the reserved bread pieces around the bowl to toast lightly.
08 - Remove from the oven and serve the dip warm with the toasted bread pieces or breadsticks.