01 - Preheat the oven to 350°F for even baking.
02 - Heat a medium skillet over medium heat with a little oil. Sauté the finely chopped onion until translucent, approximately 3 minutes. Add minced garlic and cook for 1 minute until fragrant.
03 - In a large mixing bowl, combine the softened cream cheese, sour cream, mayonnaise, grated Parmesan, shredded mozzarella, salt, black pepper, and nutmeg. Mix until fully incorporated and smooth.
04 - Add the sautéed onion and garlic to the cheese mixture. Fold in the well-drained spinach until evenly distributed throughout the base.
05 - Transfer the spinach mixture to a small ovenproof baking dish or spoon into a hollowed-out sourdough bread boule if preparing as a bread bowl presentation.
06 - Bake for 20 to 25 minutes until the dip is hot and bubbly with a light golden brown surface on top.
07 - Serve warm alongside sliced sourdough bread or chunks from the bread bowl for dipping while hot and creamy.