In a large pot, heat the oil. Add the white and tender green parts of the scallions, along with the leeks, fennel and onion; season with salt and white pepper. Cook over low heat, stirring, until the vegetables begin to soften, 15 minutes. Add the wine and boil over high heat until reduced to a few tablespoons, 12 minutes. Add the water and cream and bring to a simmer. Cook until the vegetables are very tender and pale green, 15 minutes. Add the scallion green tops and cook just until softened, 2 minutes.