01 - Preheat oven to 350°F.
02 - In a mixing bowl, combine cream cheese, sour cream, and mayonnaise. Beat the mixture until smooth and creamy.
03 - Stir in the thawed spinach, Parmesan cheese, mozzarella cheese, minced garlic, diced onion, salt, pepper, and nutmeg until well combined.
04 - Slice the top off the sourdough loaf and hollow out the center, leaving a 3/4 inch thick shell. Reserve the removed bread for dipping.
05 - Spoon the spinach mixture into the bread bowl. Place the filled bowl on a baking sheet and bake uncovered for 30 minutes, or until bubbling and golden.
06 - During the last 10 minutes of baking, arrange the reserved bread pieces around the bowl on the sheet to toast lightly.
07 - Remove from the oven. Serve the dip warm with toasted bread cubes or breadsticks.