Place a large pot over medium heat and add the butter. When the butter has melted add the onion and a sprinkling of salt and cook for 3-4 minutes until the onion has softened. Stir often and try to sweat the onions, not brown them.Add the sweet potato chunks and sauté in the onions for 2-3 minutes. Add the pears and sauté for another 2 minutes.Add the stock to the pot and simmer for 20-30 minutes until the sweet potatoes and pears are soft.Whiz with a stick blender until smooth and then test for seasoning. If your stock is salty you might only need some pepper but only you will be able to tell if you like it.Reheat almost to the boil and stir in the yogurt.Serve with a garnish of fresh chopped mint, croutons or chunks of roasted sweet potato.If you need more than two servings, this recipe is easy to double or triple.