In a large pot, heat butter or oil over medium heat.
Add leeks and sweat, stirring often, until soft, about 6 minutes. Add potato, vermouth and chicken stock and bring up to a simmer.
Cook until potatoes are tender, about 15 minutes. Add peas and sprigs of herb (tie them with a string to keep them together).
Bring just up to a simmer, remove herbs and purée soup until smooth. Strain for a smoother soup (optional) then season to taste and serve immediately garnished with sour cream or crème fraiche.
Alternatively, this soup is lovely served chilled topped with baby shrimp, crab or chopped avocado.