01 - Preheat oven to 375°F. Lightly grease a 9x13-inch casserole dish with olive oil or cooking spray and set aside.
02 - Heat olive oil in a large skillet over medium heat. Add the diced onion and sauté until translucent and softened, about 4 minutes. Add the minced garlic and cook for an additional 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 from heat and set aside to cool slightly.
04 - In a large mixing bowl, combine the softened cream cheese, sour cream, chicken broth, kosher salt, black pepper, and Italian herb blend. Stir until smooth and well incorporated.
05 - Fold the shredded chicken, chopped artichoke hearts, sautéed spinach mixture, half of the mozzarella cheese, and half of the Parmesan cheese into the creamy base. Mix thoroughly until evenly distributed.
06 - Transfer the mixture to the prepared casserole dish, spreading it into an even layer. Top with the remaining mozzarella and Parmesan cheeses, then sprinkle the gluten-free breadcrumbs evenly over the surface.
07 - Bake on the center rack for 25 to 30 minutes, or until the cheese is melted and bubbly and the breadcrumb topping is golden brown.
08 - Remove from the oven and let the casserole rest for 5 minutes to allow the filling to set before slicing and serving.