In a medium saucepan over medium heat, melt the butter. Add onions and saute until tender, about 3 to 5 minutes. Don't allow the onions to brown at all. Carefully add the vermouth and stir with a wooden spoon. Simmer until the vermouth is reduced by half, about 3 minutes. Strain clams and reserve juice. Add clam juice and reduce a little, and then add cream and reduce by half. Add salt, pepper, and sugar to taste. Add potato soup and bring to a simmer. Add clams and cook just until clams are warm. Taste for seasoning, and add truffle oil, if using. Serve in sourdough bowl. Top with chopped parsley.
Recipe courtesy of Sandra Lee