Melt the butter in a large, deep pan over medium heat. Sauté onions for 2-3 minutes, or until softened but not brown. Add the diced sweet potato and diced pear, and sauté for 3-4 minutes. (If using canned pears, only sauté the diced sweet potato at this stage and add the pears later.)
Add the broth to the pot. Bring to a boil and simmer gently for 20-25 minutes, or until the sweet potato (and fresh pears) are soft. If including wine in your soup, add this in the last 10 minutes simmering. Remove from heat and cool for about 30 minutes. If using canned pears, add them now, minus the juice, which you should save. You don’t need to dice the pears, as the next step will take care of that.
Add the mixture to a blender or food processor, and blend until smooth, about 1-2 minutes. (If your blender is not big enough to fit the entire batch, divide the soup into two containers and blend each separately.) Adjust the thickness and flavor of the soup to your preference by adding either more wine, more pears, or some pear juice.
Return the soup to the pan and gently reheat it, without bringing it to a boil. Ladle into bowls and swirl in some yogurt or creme fraiche. Garnish with mint or parsley.