Preheat the oven to 375°F (190°C). Grease a 9x13-inch baking dish.
In a large mixing bowl, combine the shredded chicken, chopped artichokes, and drained spinach.
In a separate bowl, mix together mayonnaise, sour cream, Parmesan cheese, 1 cup of mozzarella cheese, minced garlic, dried oregano, onion powder, salt, and pepper.
Add the creamy mixture to the chicken and vegetables, stirring until everything is well-coated.
Transfer the mixture to the prepared baking dish, spreading it evenly. Top with the remaining cheese.
If desired, sprinkle breadcrumbs over the top for a crunchy finish.
Bake in the preheated oven for 35-40 minutes or until the casserole is bubbly and golden brown on top.
Remove from the oven and let it rest for a few minutes before serving.