Preheat your oven to 350°F (175°C) and lightly grease a 9-inch baking dish.
Squeeze all excess moisture from the thawed spinach using a clean kitchen towel or several layers of paper towels. This crucial step prevents a watery dip.
In a large mixing bowl, beat the cream cheese until smooth and fluffy, about 1 minute.
Mix in the mayonnaise, sour cream, 3/4 cup of mozzarella, and 1/4 cup of Parmesan cheese until well combined.
Fold in the drained spinach, chopped artichoke hearts, minced garlic, red pepper flakes, salt, and pepper until evenly distributed throughout the mixture.
Stir in the fresh lemon juice to brighten the flavors and balance the richness.
Transfer the mixture to your prepared baking dish and spread it evenly with a spatula.
Sprinkle the remaining 1/4 cup mozzarella and 1/4 cup Parmesan cheese over the top for that perfect golden crust.
Bake for 25-30 minutes until the dip is bubbling around the edges and the top is golden brown.
Let the dip rest for 5-10 minutes before serving to allow it to set and cool slightly.