01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish with olive oil or cooking spray.
02 - In a large skillet, heat olive oil over medium heat. Add the diced onion and cook until softened, about 3 minutes. Add the minced garlic and sauté for 1 additional minute until fragrant.
03 - Add the chopped spinach to the skillet and cook, stirring occasionally, until fully wilted, about 2 minutes. Remove the skillet from heat and set aside.
04 - In a large mixing bowl, combine the softened cream cheese and sour cream. Stir until smooth and well blended.
05 - Fold in the shredded chicken, chopped artichoke hearts, spinach mixture, half of the mozzarella cheese, Parmesan cheese, dried oregano, dried basil, salt, black pepper, and red pepper flakes if using. Mix until everything is evenly incorporated.
06 - Transfer the mixture to the prepared baking dish and spread it 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 the oven and let the casserole rest for 5 minutes before serving to allow the filling to set.