01 - Set the oven temperature to 350°F (180°C).
02 - In a mixing bowl, blend cream cheese, sour cream, and mayonnaise until smooth and creamy.
03 - Add thawed spinach, Parmesan, mozzarella, minced garlic, diced onion, salt, pepper, and nutmeg; mix thoroughly.
04 - Cut the top off the sourdough loaf and hollow out the center, leaving a shell approximately 3/4 inch thick; reserve 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 bubbling and golden on top.
07 - During the last 10 minutes of baking, arrange reserved bread pieces around the bowl to toast lightly.
08 - Remove from oven and serve immediately with toasted bread cubes or breadsticks.