In a large bowl, combine the spinach, chicken, Cheddar, onion, cornstarch, salt, pepper, soy sauce, soup, sour cream, mushrooms and mayonnaise. Put a layer of noodles in the bottom of a greased 9-by-13-by-2-inch casserole dish. Spread half of the spinach mixture over the noodles. Put another layer of noodles over the mixture and cover with the remaining spinach mixture. Sprinkle the Parmesan over the casserole. Then sprinkle the pecans on top. Bake for 1 hour. Let the casserole sit for 15 minutes before serving.