Boil the potatoes in a large stock pot until they are soft. Once they are done, drain in the sink with a colander and let sit for 5 minutes.
While the potatoes are boiling dice your onion and cut the butter into small chunks and set aside.
Return the potatoes to the stock pot and mash with a potato masher. Mash to the desired consistency.
Once at the consistency wanted add both containers sour cream, diced onion, butter, salt and pepper. Use a large spoon to mix everything together until blended and smooth. If you need to make them smoother you can use the potato masher to mash them some more.