Heat butter in a large saucepan or soup pot. Add leeks, sweet potatoes, and celery and sauté over low heat until leeks are wilted and lightly golden.

Add Light Chicken Stock or broth, sage, salt, and pepper. Cover and simmer for 30 to 40 minutes, or until potatoes are very tender.

Purée soup in a food processor. Depending on the size of your processor, you may have to do this in 2 or more steps.

Return soup to pot and bring to a boil. Reduce to a simmer and correct seasoning. Add butter and whisk until it is completely blended into soup. Add diced turkey.

Pour soup into a tureen or ladle into individual soup bowls.