Prepare the lime creme: Stir the juice and grated rind of 2 limes into sour cream to attain a pourable consistency. Refrigerate until ready to use. You may wish to pour the lime cream into a plastic squirt bottle in order to create a pattern atop the surface of the soup for presentation.

In a small skillet, saute onion in butter until soft. Puree sweet potatoes, onion and 1 cup of the stock in a blender or food processor. Place pureed mixture in a saucepan. Add remaining stock, jalapenos, salt and pepper. Cover and simmer over low heat for about 30 minutes. Stir the soup and ladle into bowls. Swirl about 1 tablespoon of the lime creme into each bowl, and garnish with cilantro. Serves 8.