01 - Preheat the oven to 350°F.
02 - In a medium skillet over medium heat, sauté the onion in a little oil until translucent, about 3 minutes. Add garlic and cook for 1 minute more.
03 - In a large mixing bowl, combine sautéed onion and garlic, cream cheese, sour cream, mayonnaise, Parmesan, mozzarella, salt, pepper, and nutmeg (if using). Mix until smooth.
04 - Fold in the well-drained spinach until evenly incorporated.
05 - Transfer the mixture to a small ovenproof baking dish or, if using, into a hollowed-out sourdough bread boule.
06 - Bake for 20–25 minutes, until hot and bubbly with a light golden top.
07 - Serve immediately with sliced sourdough bread or chunks of the bread bowl for dipping.