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 diced onion and cook until softened, about 3 minutes. Add minced garlic and sauté for 1 additional minute until fragrant.
03 - Add the chopped spinach to the skillet and cook, stirring occasionally, until 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, Parmesan cheese, oregano, basil, salt, black pepper, and red pepper flakes if using. Mix until all ingredients are evenly incorporated.
06 - Spread the chicken mixture evenly into the prepared 9x13-inch baking dish. Sprinkle the remaining mozzarella cheese uniformly over the top.
07 - Bake uncovered for 30 minutes, or until the casserole is 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.