Two granny smith apples
A bit of butter
Salt & pepper
Any other seasonings you want to throw in
Bake the sweet potatoes until they're soft.
Chop the onion and saute it in some butter. Peel and thinly slice the apple and toss it in too. Cook until the onions are translucent and the apple is tender. Scoop out the sweet potatoes and add to the onion/apple mixture. Add about six cups of water and simmer for about ten minutes, or until the soup is heated through. Add salt, pepper, and other seasonings to taste. Puree the whole thing with an immersion blender.
This recipe is simple, forgiving, and flexible. You don't really need to measure anything, and you can season it just about any way you want-- try curry powder. You could use chicken stock instead of water, for a richer flavor. It works well with butternut squash too.