In a large pan, bring salted water to boil. Add squash and onion. Return to boil and cook 5 minutes. Drain well. In a large bowl, combine soup and sour cream, stir in carrot, and then fold in squash and onion. Pour package of stuffing mix into a 13x9x2” baking dish. Drizzle with melted butter (or chicken broth), then fluff with a fork until well moistened. Spread stuffing evenly in bottom of the dish. Spoon on veggie mixture. Top with cheddar cheese. Bake at 350° for about 30 minutes, or until cheddar is lightly browned.