Preheat the oven to 350 degrees.

In a medium saucepan, bring 3 cups of salted water to a boil. Add squash and onions and boil for about 5 minutes. Drain.

In a large bowl, combine the soup and the sour cream, mixing well. Stir in the carrots. Gently fold in the squash and onions.

In another bowl, mix the butter with the stuffing mix until crumbly. Spread half of the stuffing in a 12x7x2 inch baking dish. On top of this, spread the squash mixture from edge to edge and cover with the remaining stuffing. Top with shredded parmesan.

Bake for 25 minutes.