Peel 2 lb. parsnips and 1 sweet potato and cut into 1-inch pieces. Bring parsnips, sweet potato, 1 chopped onion, 1 (2-inch) piece peeled fresh ginger and 1 quart chicken broth to a boil. Simmer for 25 minutes. Blend half of mixture with 1 cup heavy cream. Transfer to a bowl. Blend remaining mixture with 1 cup water. In a saucepan, combine pureed mixtures; stir in 1/2 cup water and 1/2 tsp. kosher salt. Season with black pepper; warm over medium heat. Sprinkle with 1 cup chopped toasted walnuts and 1/4 cup minced chives.