Start by slicing your zucchini into approx. 1/4 inch slices and rough chopping your onion. Place in a large pot with salt to tatse and a small amount (approx. 1/2 cup) of water just to start the veggies cooking (they will produce alot of liquid on their own once they are cooked). Bring to a boil on med-high heat until they are tender. This should take about 15-20 minutes. Then strain off excess liquid and add the veggies back to your pot. Add the soup and sour cream to the veggies, as well as whatever salt & pepper you would like. Mix well and set aside.
Next comes the topping. You will use the stuffing on the bottom and top of this casserole. Start boiling HALF as much water and TWICE as much butter (yes real butter!) as the package directions state. Preparing it this way will give you a little "crunch" on your topping. Once the water & butter has reached a boil, remove from the heat and add the stuffing mix, mix well and cover for a couple minutes just to let the liquid soak up well.
Now to put it all together. Take half of the prepared stuffing mix and spread it across the bottom of any large rectangular baking dish. Then pour in your zucchini filling (make sure you have enough room for the rest of your stuffing mix on top, you may have some filling left over), then just spread the remaining stuffing on top.
Then just pop it in the oven for about 30 minutes or until it is bubbly around the edges!