01 - Preheat your oven to 350°F. Using a serrated knife, slice off the top quarter of the sourdough loaf. Carefully hollow out the center, leaving a shell about 1 inch thick. Set aside the removed bread chunks to use as dippers.
02 - In a large mixing bowl, combine the softened cream cheese, sour cream, mayonnaise, Parmesan, mozzarella, minced garlic, diced onion, black pepper, salt, and nutmeg. Beat with a spoon or electric mixer until completely smooth and well incorporated.
03 - Add the squeezed-dry chopped spinach to the cheese mixture. Fold gently but thoroughly until the spinach is evenly distributed throughout the creamy base.
04 - Spoon the spinach mixture into the hollowed bread bowl, packing it lightly. Place the filled bowl on a baking sheet and loosely cover with the reserved bread top. Bake for 30 minutes until the dip is heated through and bubbling around the edges.
05 - Remove from the oven and discard or set aside the bread top. Sprinkle the hot dip generously with fresh chives or parsley. Serve immediately while warm, accompanied by the reserved bread pieces, raw vegetables, or crackers.