Scrub the sweet potato to remove any dirt. Peel skin if you like and cut into cubes (I used purple skinned Japanese sweet potatoes so left the skin on because I like the color and flavor). Slice the onion thin.

Heat the butter or olive oil in a medium pot. Add the onions and saute on medium low until soft and translucent.

Add the sweet potatoes, salt, cinnamon and nutmeg and stir for a minute to coat in oil and distribute the spices.

Pour in enough broth to just cover everything. Bring to a boil, turn to low heat and simmer with the lid on for about 15 minutes until the sweet potatoes are soft.

After the sweet potatoes are soft, blend the soup with a stick blender or mixer of some sort until smooth and thick. If it's too thick, add a little more broth - or - to make it a little creamier add 100 to 200 ml milk and mix well.