01 - Preheat the oven to 350°F (180°C) to ensure proper baking temperature.
02 - Heat a medium skillet over medium heat with a small amount of oil. Sauté the finely chopped onion until translucent, approximately 3 minutes. Add minced garlic and continue cooking for 1 minute until fragrant. Remove from heat and set aside.
03 - In a large mixing bowl, combine softened cream cheese, sour cream, mayonnaise, grated Parmesan, shredded mozzarella, salt, black pepper, and ground nutmeg. Mix thoroughly until completely smooth and well incorporated.
04 - Add the sautéed onion and garlic mixture to the cheese base. Fold in the thoroughly drained spinach, mixing gently until evenly distributed throughout the dip mixture.
05 - Spoon the dip mixture into a small ovenproof baking dish. For presentation, alternatively use a hollowed-out sourdough bread boule as the serving vessel.
06 - Place the dish in the preheated oven and bake for 20 to 25 minutes. The dip is ready when hot and bubbly with a light golden color on top.
07 - Remove from the oven and serve warm while still creamy. Accompany with sliced sourdough bread or chunks from the bread bowl for dipping.