01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish with butter or nonstick spray.
02 - Heat olive oil in a large skillet over medium heat. Add chopped onion and sauté until translucent, about 2 to 3 minutes. Add minced garlic and cook for 1 additional minute until fragrant.
03 - Stir fresh spinach into the skillet and cook until wilted. Remove from heat and let cool slightly.
04 - In a large mixing bowl, combine softened cream cheese, sour cream, mayonnaise, milk, Parmesan cheese, and 1 cup of mozzarella. Blend until smooth and well incorporated.
05 - Fold sautéed vegetables, shredded chicken, chopped artichoke hearts, salt, black pepper, paprika, and red pepper flakes into the creamy base. Stir until evenly distributed.
06 - Spread the chicken and vegetable mixture evenly into the prepared baking dish. Sprinkle the remaining 1 cup of mozzarella cheese uniformly 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 oven and let rest for 5 minutes. Garnish with additional Parmesan cheese or chopped fresh parsley if desired before serving.