Chop the onion and celery. Melt butter and add onion, celery, garlic. Peel and chop the sweet potatoes. Add to the vegetables with the broth and the spices. Cover and cook over medium heat for 20 minutes or until the sweet potatoes are very tender. Mash the potatoes or puree them in a blender (in batches). Add the cream and mix well. Add salt and pepper to taste. You can cool the soup and refrigerate to serve later.

Serve the soup hot. Top with chunks of ripe avocado.