Preheat your oven to 350°F (175°C). This will ensure your dip cooks evenly.
In a large mixing bowl, combine the mayonnaise, sour cream, and Dijon mustard. Stir well until all ingredients are fully blended.
Add the salt and ground pepper to the bowl. Mix until the spices are evenly distributed throughout the creamy base.
In a skillet over medium heat, sauté the finely chopped onion until it becomes translucent and fragrant, about 5 minutes.
In the mixing bowl, add the chopped spinach and drained, chopped artichoke hearts. Stir to combine all the ingredients thoroughly.
Fold in 9 ounces of the shredded mozzarella cheese and the grated Parmesan cheese into the mixture. Reserve the remaining mozzarella for topping later.
Pour the dip mixture into your prepared baking dish, spreading it out evenly.
Sprinkle the reserved mozzarella cheese over the top of the dip.
Place the dish in the preheated oven and bake for 25-30 minutes or until the dip is bubbly and the cheese is golden brown.
Once baked, remove the dip from the oven and let it cool for a few minutes. Serve warm with tortilla chips, pita bread, or fresh veggies.
