01 - Set oven temperature to 350°F.
02 - In a large mixing bowl, blend cream cheese, sour cream, and mayonnaise until smooth.
03 - Mix drained spinach, mozzarella, Parmesan, minced garlic, onion powder, black pepper, salt, and nutmeg into the creamy base until fully combined.
04 - Transfer mixture to an oven-safe baking dish and level the surface.
05 - Bake for 20 to 25 minutes until mixture is hot, bubbly, and lightly golden on top.
06 - While baking, slice off top of sourdough loaf and hollow out the interior, preserving the removed bread pieces.
07 - Brush inside of bread bowl and reserved pieces with olive oil and toast in oven for 5 to 7 minutes until lightly crisp, if desired.
08 - Pour hot spinach dip into bread bowl and serve immediately with toasted bread pieces and additional sourdough slices.