Place the onion, sweet potatoes, and stock in a large saucepan and bring to a boil over high heat. Reduce the heat, cover, and cook until the sweet potatoes are soft, about 25 minutes.

Remove from the heat and cool slightly. Transfer to a blender and puree in batches until smooth.

Return the soup to the saucepan, and place over medium heat. Add the milk and heat through. Add the lemon juice and nutmeg and season with salt and pepper to taste.