Step 1

Cook the onions, celery, leeks, and garlic: Melt the butter in a large, thick-bottomed pot over medium-high heat. Add the chopped onions and sauté for 3 to 4 minutes, then add the celery and leeks and sauté for 5 more minutes until softened. Add the garlic and cook a minute more.

Step 2

Make and simmer the soup: Add the sweet potatoes, chicken stock, cinnamon stick, and nutmeg. Increase the heat to high and bring to a boil. Reduce to a simmer, and simmer uncovered until the sweet potatoes can be easily pierced with a fork, about 20 minutes.

Step 3

Blend the soup: Remove the cinnamon stick. Use an immersion blender to purée the soup or work in batches and purée the soup in a standing blender.

Step 4

Finish the soup: Add the cream and milk to the soup. Heat on medium until the soup is heated through. Add salt and pepper to taste.Sprinkle with more black pepper and swirl some water-thinned sour cream or plain yogurt over the soup to serve.