01 - Preheat oven to 375°F. Lightly grease a 9x13-inch casserole dish with olive oil or cooking spray.
02 - Heat olive oil in a large skillet over medium heat. Add the diced onion and cook until softened and translucent, about 4 minutes. Add the minced garlic and stir for 1 minute until fragrant.
03 - Add the chopped spinach to the skillet and cook, stirring occasionally, until wilted if using fresh, or heated through if using frozen. Remove the skillet from heat and set aside.
04 - In a large mixing bowl, combine the softened cream cheese, sour cream, chicken broth, kosher salt, black pepper, and Italian seasoning. Stir until smooth and well blended.
05 - Fold the shredded chicken, chopped artichoke hearts, and spinach mixture into the creamy base. Add half of the mozzarella and half of the Parmesan cheese, then mix until everything is evenly distributed.
06 - Transfer the mixture into the prepared casserole dish, spreading it out in an even layer. Top with the remaining mozzarella and Parmesan cheeses, then sprinkle the gluten-free breadcrumbs evenly over the surface.
07 - Bake uncovered for 25 to 30 minutes, or until the edges are bubbling and the breadcrumb 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.