01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish with olive oil or nonstick spray.
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, tossing 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 fully blended.
05 - Fold in shredded chicken, chopped artichoke hearts, the spinach and onion mixture, half of the mozzarella, 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 across 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.