Preheat oven to 350 degrees. Dice squash and onion.
In a large frying pan, add the olive oil, squash, and onion. Cook on medium until squash is soft. To speed up the process you can cover the pan for the first few minutes. If the bottom of the pan gets too dry, add a little water.
Melt butter in a medium sized bowl in the microwave, then add stuffing mix and mix well. Set aside.
Once squash and onion are done, mix in a large bowl with soup, cheese, sour cream, salt and pepper, and half of the stuffing mix.
In an oiled baking dish (I use 9x13 but mostly any size will work) spread half of the remaining stuffing mix on the bottom. It will not completely cover the bottom. Add the casserole and then sprinkle the remaining stuffing mix on top.