In a large bowl, stir together the cream cheese, mayo, 1 cup mozzarella, 1/4 cup parmesan, the cooled chicken and onion mixture, salt, and pepper until well combined. The warmth from the chicken will help melt the cream cheese slightly. Pour the mixture into a greased 9-inch square or rectangular baking dish, smoothing out as best you can. Drizzle with the remaining marinara and then top with remaining mozzarella, parmesan, Panko, breadcrumbs and a sprinkle of Italian seasoning. Bake until golden brown and bubbly, about 20 to 25 minutes.