Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking dish.
Combine water and margarine in a saucepan and bring to a boil. Stir stuffing mix into water and margarine until moistened; cover, remove from heat, and let stand 5 minutes. Fluff with a fork.
Spread cooked chicken into bottom of the prepared baking dish and spread frozen vegetables over chicken. Top vegetables with American cheese slices in a layer. Mix cream of mushroom soup, garlic powder, marjoram, thyme, and sage in a bowl and spread over cheese slices. Sprinkle stuffing over top of casserole in an even layer.
Bake in the preheated oven until casserole is bubbling and browned, about 1 hour.