- In a large saucepan over medium heat, melt the butter.
- Add the bacon and cook until lightly browned, about 5 minutes. Add shallots, apple, celery root, and mushrooms and cook for another 10 minutes.
- Add the chestnuts and cook for another 2 minutes. Add the cognac and Armagnac and cook until reduced by half.
- Add chicken stock, thyme, bay leaf, and sage and cook for 20 minutes or until the vegetables are tender.
- Purée soup in a blender until creamy and smooth. Strain the purée through a fine mesh strainer (chinois) into another saucepan. Return the soup to a burner over low heat and bring to a simmer.
- Add the cream and season with salt, sugar, and pepper.
- Ladle soup into individual bowls and garnish with a tablespoon of sour cream and fresh (or fried) sage leaf. Serve immediately.