01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish with cooking spray or butter.
02 - Heat olive oil in a large skillet over medium heat. Add the chopped onion and sauté until translucent, about 2 to 3 minutes. Add minced garlic and cook for 1 additional minute until fragrant.
03 - Stir in the fresh spinach and cook until wilted, approximately 2 minutes. Remove the skillet from heat and allow the vegetables to cool slightly.
04 - In a large mixing bowl, combine the softened cream cheese, sour cream, mayonnaise, milk, Parmesan cheese, and 1 cup of mozzarella cheese. Stir until smooth and well blended.
05 - Fold the sautéed vegetable mixture, shredded chicken, chopped artichoke hearts, salt, black pepper, paprika, and red pepper flakes into the creamy base. Mix until all ingredients are evenly distributed.
06 - Transfer the mixture to the prepared baking dish and spread it into an even layer. Sprinkle the remaining 1 cup of mozzarella cheese uniformly over the top.
07 - Bake uncovered for 30 to 35 minutes, or until the casserole is bubbly around the edges and the cheese topping is golden brown.
08 - Remove from the oven and let the casserole rest for 5 minutes before serving. Garnish with additional Parmesan cheese or fresh chopped parsley if desired.