01 - Set the oven temperature to 350°F and allow it to fully preheat.
02 - Slice off the top of the sourdough loaf and carefully hollow out the center, keeping a ¾ to 1-inch thick shell intact. Cube the removed bread for serving.
03 - In a large bowl, blend cream cheese, sour cream, mayonnaise, Parmesan, mozzarella, minced garlic, diced onion, salt, pepper, and nutmeg into a smooth mixture.
04 - Fold the thoroughly drained spinach into the cheese mixture until evenly distributed.
05 - Spoon the dip mixture into the hollowed sourdough loaf. Place the filled loaf and the bread cubes on a baking sheet.
06 - Bake in the preheated oven for 25 minutes or until the dip bubbles and the top turns golden.
07 - Garnish the dip with chopped fresh chives, if using, and serve immediately with the toasted bread cubes.