01 - Preheat the oven to 350°F (180°C).
02 - In a large mixing bowl, blend cream cheese, sour cream, and mayonnaise until smooth and uniform.
03 - Fold in the well-drained spinach, shredded mozzarella, grated Parmesan, minced garlic, onion powder, black pepper, salt, and nutmeg until fully combined.
04 - Transfer the mixture into an oven-safe baking dish, spreading evenly and smoothing the top.
05 - Bake for 20 to 25 minutes until the mixture is heated through, bubbling, and the surface turns lightly golden.
06 - Slice off the top of the sourdough loaf and hollow out the center to form a bowl, reserving the removed bread pieces.
07 - Optionally brush the interior of the bread bowl and reserved pieces with olive oil and toast in the oven for 5 to 7 minutes until lightly crisp.
08 - Spoon the hot spinach mixture into the bread bowl and serve immediately alongside the toasted reserved bread pieces and additional sliced sourdough.