Pour chicken stock into a large sauce pan and turn heat on medium high. Add onions, diced potatoes, and butter. Cook for 15 minutes or until potatoes are tender.
If you don't have an emulsion blender, pour small amounts into a regular blender and blend the ingredients until smooth. Pour the ingredients back into the pot and blend the rest of the soup until all the soup is blended together.
Turn the soup on low heat and continue to stir. Stir in the heavy cream, sour cream, and half of the cheddar cheese. Season to taste.