01 - Set the oven temperature to 350°F (180°C).
02 - Cut off the top of the sourdough loaf and hollow out the center, leaving a ¾ to 1-inch thick shell. Cube the removed bread for serving.
03 - In a large mixing bowl, blend cream cheese, sour cream, mayonnaise, Parmesan, mozzarella, garlic, onion, salt, pepper, and nutmeg until smooth.
04 - Fold the drained spinach into the cheese mixture until fully combined.
05 - Spoon the dip mixture evenly into the hollowed sourdough bowl. Arrange the bread cubes alongside on a baking sheet.
06 - Place the baking sheet in the oven and bake for 25 minutes, until the dip is bubbling and the top is golden brown.
07 - Sprinkle chopped fresh chives over the warm dip if desired. Serve immediately with bread cubes for dipping.