In a pot, melt the butter in the olive oil over medium-low heat. Add the onion, carrot, and celery, a big pinch of salt, and sweat for 20 minutes until the onions are translucent. The vegetables should not color. Season with nutmeg, to taste.
Add the ginger and cook for a few more minutes. Next, toss in the sweet potatoes, and add the cinnamon, clove, and allspice. Allow the mixture to cook for a few minutes. Now, add enough stock to cover everything in the pot. (Don’t go crazy with it — you can always add more later to thin it out.) Add a bit more salt, bring to a boil over high heat, lower to a simmer, and cover.
Cook for 20 minutes, or until the sweet potato is so soft you can mash it against the side of the pot with a wooden spoon. (The softness of the sweet potato is essential, so if you’re unsure, cook it a few minutes longer.)
Remove from the heat, and pour it into a blender. (You may have to blend the soup in batches; do not fill the blender more than halfway.) Put the lid on but keep the hole open (otherwise it might explode) and cover with a kitchen towel as you blend your soup. Let it blend for a good minute, you want it extra creamy.
If desired, you can add some cream and pure maple syrup, to taste. Ladle into bowls, and serve.