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 finely 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, about 2 minutes. Remove from heat and let the vegetable mixture cool slightly.
04 - In a large mixing bowl, combine the softened cream cheese, sour cream, mayonnaise, milk, grated Parmesan, and 1 cup of shredded mozzarella. Stir until smooth and well blended.
05 - Fold in the sautéed vegetable mixture, shredded chicken, chopped artichoke hearts, salt, black pepper, paprika, and red pepper flakes if using. Mix until all components are evenly distributed throughout the creamy base.
06 - Transfer the mixture to the prepared baking dish and spread it into an even layer. Sprinkle the remaining 1 cup of mozzarella cheese evenly across the top.
07 - Bake uncovered for 30 to 35 minutes, 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 or freshly chopped parsley if desired.