Peel the sweet potatoes and carrots and cut into pieces. Cook in salted water until tender.
Dice the onion and lightly sweat it in a saucepan. Drain the sweet potatoes and carrots and add to the onions. Add the vegetable stock and bring to the boil briefly.
Take the pot off the stove and purée the soup finely with the blender. Season to taste with salt, pepper and nutmeg.
Serve the soup and gently stir in 2 teaspoons of crème fraîche. Cut the baguette into slices and serve with the soup.