Preheat oven to 350 degrees F. Wash and cut squash into bite-sized pieces.
Place squash, onion, and chicken broth into a stovetop pot. Add enough water to cover squash. Bring to a boil and continue to boil for 20 minutes or until squash is fork tender.
Drain liquid and toss in the soup and sour cream and stir until squash is coated.
Mix the butter and stuffing mix and spoon about half of the mixture in the bottom of a casserole dish. Top with the squash and then spoon the rest of the stuffing on top of the squash. Cover dish with aluminum foil.
Bake at 350 degrees F. for about 20 minutes and then uncover and bake for 5 more minutes.
If you'd like, you can top your casserole with grated sharp cheddar cheese!