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 softened and translucent, 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 the pan 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 dried Italian herbs. Stir vigorously until the mixture is smooth and well blended.
05 - Fold the shredded chicken, chopped artichoke hearts, spinach mixture, half of the mozzarella, and half of the Parmesan into the cream base. Mix thoroughly until all ingredients are evenly distributed.
06 - Transfer the chicken and spinach mixture into the prepared casserole dish, spreading it into an even layer. Scatter the remaining mozzarella and Parmesan over the top, then finish with an even layer of breadcrumbs.
07 - Bake on the center rack for 25 to 30 minutes, or until the cheese is melted and bubbly and the breadcrumb topping turns a deep golden brown.
08 - Remove from the oven and let the casserole rest for 5 minutes before scooping into portions. This allows the filling to set for cleaner serving.