Winter Root Vegetable Stew

Name: Winter Root Vegetable Stew
Ingredients: 3 pieces dried lotus root; 6-inch piece kombu, wiped clean; 1/3 cup thinly sliced burdock root; 3 tablespoons minced ginger; 5 cups finely diced rutabaga; 2½ cups medium diced daikon; 2 cups medium diced celery root; 1½ cups coarsely chopped onions; 1 1/3 cups sliced carrots; ¾ cup sliced parsnips; 4 cups water; ¼ cup shoyu, or to taste; 2 tablespoons kuzu or arrowroot; ½ cup cold water; 3-4 chopped scallions for garnish