01 - Set the oven to 350°F to prepare for baking.
02 - Combine cream cheese, sour cream, and mayonnaise in a large bowl until smooth.
03 - Fold in the well-drained spinach, shredded mozzarella, grated Parmesan, minced garlic, onion powder, black pepper, salt, and nutmeg until evenly blended.
04 - Transfer the mixture into an oven-safe baking dish and level the surface.
05 - Bake for 20 to 25 minutes until the dip is heated through, bubbly, and lightly golden on top.
06 - Slice the top off the sourdough loaf and hollow out the interior to create a bowl, reserving the removed bread pieces.
07 - Brush the inside of the bread bowl and reserved pieces lightly with olive oil, then toast in the oven for 5 to 7 minutes until crisp.
08 - Spoon the hot spinach dip into the bread bowl and serve immediately with toasted bread pieces and additional sourdough slices.