Bring a large pot of water to a boil. Preheat oven to 350°F.

Once the water is boiling, add squash and cook until soft.

Drain well, then mash with a fork.

In a large skillet, melt 2 tablespoons of the butter. Add onion and sauté until soft. Add squash, cream of chicken soup, and sour cream. Stir well to combine.

Melt the remaining 4 tablespoons of butter in a large bowl. Add the stuffing and mix well.

Add ⅔ of the stuffing mixture to the squash mixture and stir to combine. Spread into a 9x13-inch baking dish, then top with the remaining ⅓ of the stuffing mixture.

Add to mixture, reserving ⅓ of stuffing; top with remaining stuffing.

Bake in the preheated oven for 30 minutes.