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 diced onion and cook until softened, about 3 minutes. Add minced garlic and sauté for 1 additional minute until fragrant.
03 - Add chopped spinach to the skillet and cook, stirring occasionally, until wilted, about 2 minutes. Remove from heat and set aside.
04 - In a large mixing bowl, combine softened cream cheese and sour cream. Stir until smooth and well blended.
05 - Fold in shredded chicken, chopped artichoke hearts, the spinach-onion mixture, half of the mozzarella cheese, Parmesan cheese, oregano, basil, salt, black pepper, and red pepper flakes if using. Mix until evenly incorporated.
06 - Transfer the mixture to the prepared baking dish and spread into an even layer. Sprinkle the remaining mozzarella cheese evenly over the top.
07 - Bake uncovered for 30 minutes, or until the edges are bubbling and the cheese topping is golden brown.
08 - Remove from oven and let rest for 5 minutes before serving to allow the casserole to set.