01 - Set the oven temperature to 350°F (180°C) to prepare for baking.
02 - Slice off the top of the sourdough loaf and scoop out the interior, leaving a shell approximately 0.8 to 1.2 inches thick; cube the removed bread for dipping.
03 - In a large bowl, blend cream cheese, sour cream, mayonnaise, Parmesan, mozzarella, minced garlic, diced onion, salt, pepper, and nutmeg until the mixture is smooth and uniform.
04 - Fold the well-drained chopped spinach into the cheese mixture until evenly incorporated.
05 - Spoon the combined mixture into the hollowed sourdough loaf; arrange the bread cubes around it on a baking sheet.
06 - Bake in the preheated oven for 25 minutes, until the dip is bubbling and golden on the surface.
07 - Sprinkle chopped chives over the baked dip if desired, then serve immediately with the bread cubes for dipping.