1 1/2poundssweet potatoes, cut into 1-inch pieces(about 2 large orange-flesh sweet potatoes)
Melt the butter in a large, stock pot over medium-high heat. Add the chopped onions and sauté for 3 to 4 minutes, then add the celery and sauté for 5 more minutes until softened. Then add the garlic and cook a minute more.
Add sweet potatoes, vegetable stock, cinnamon stick, and nutmeg. Increase the heat to high and bring to a boil. Reduce heat to a simmer, and simmer uncovered until the sweet potatoes can be easily pierced with a fork, about 20 minutes.
Remove cinnamon stick. Use an immersion blender to purée the soup or work in batches and purée the soup in a standing blender.
Add the cream and milk to the pureed sweet potato soup, and heat on medium until the soup is heated through. Add salt and pepper to taste.