1) In a medium bowl, whisk together the soymilk and cream cheese. Set aside.
2) Sauté the onion and celery in 2 tablespoons of vegan butter over medium-low heat until soft and lightly browned. Add the sage and "ham" and sauté until fragrant, about 1 minute. Add the rest of the vegan butter and flour to make a roux. Once the flour has been lightly toasted, add the water, dashi seasoning, and potatoes, and bring to a boil. Turn down to simmer and simmer, stirring often, until the potatoes are soft, about 10 minutes.
3) Add the soymilk/cream cheese mixture and oyster mushrooms. Return to a boil and simmer for about 10 more minutes, until the mushrooms are soft and the soup is thick. Add salt to taste, and garnish with chives.
For an even "fishier" flavor, you can add flaked dulse to taste at any point in cooking.
* recipe for home-made dashi stock: http://japanesefood.about.com/od/soup/r/konbudashi.htm [1]. If using this, use the same amount of kelp but only 2.5 cups water (not 4 cups as stated) to make a more concentrated broth.
Source of recipe: I wrote this recipe for the vegan chef battle :)