Spring Pea Soup with Mint

In a large pot, heat the butter or olive oil over medium heat. Add the leeks and sweat them until soft, about 6 minutes, stirring often. Add the potato, stock, and vermouth, and bring up to a simmer. Cook until the potatoes are tender, about 15 minutes. Add the peas and sprigs of herb (tying them with a string to keep them together). Bring this just up to a simmer. Remove the herbs and purée th