01 - Preheat oven to 350°F.
02 - Cut off the top quarter of the sourdough loaf using a serrated knife. Hollow out the center, leaving a 1-inch thick shell. Reserve removed bread pieces for dipping later.
03 - In a large mixing bowl, combine cream cheese, sour cream, mayonnaise, Parmesan cheese, mozzarella cheese, minced garlic, diced onion, black pepper, salt, and nutmeg. Mix until completely smooth and well blended.
04 - Add the prepared chopped spinach to the cheese mixture. Fold gently until evenly distributed throughout the dip.
05 - Spoon the spinach mixture into the hollowed bread bowl, packing it gently. Place the filled bread bowl on a baking sheet.
06 - Replace the bread top loosely over the filled bowl. Bake for 30 minutes, or until the dip is heated through and bubbles around the edges.
07 - Remove from the oven. Discard or set aside the bread top. Sprinkle generously with fresh chopped chives or parsley. Serve immediately with reserved bread chunks, vegetable crudités, or crackers.