Heat butter in large saucepan. Add onions. Cook over medium heat stirring until softened. Add sweet potatoes, seasoning and stock. Bring to a boil then cover and simmer 20 minutes or until potatoes are tender.
Purée the mixture with a hand blender or food processor. To serve reheat mixture. Add lime juice and additional stock if desired.
Serve with a swirl of sour cream or yogurt in each bowl.
The flavour improves with standing; it is ideal to prepare one or two days in advance.