Bring 8 cups broth to boil in large saucepan. Peel Jerusalem artichokes and cut into 1/3-inch thick pieces. Add to broth. Add potatoes, leeks, garlic, savory and ginger. Cover pan and simmer until vegetables are very soft, about 25 minutes. Remove from heat. Stir in marjoram; cool about 20 minutes. Working in batches, puree soup in blender until smooth. Return to same saucepan. Bring soup to simmer. Add cream. Season with salt and pepper. Simmer 5 minutes to blend flavors, thinning with more chicken broth if soup is too thick. (Can be made 1 day ahead. Cover and keep refrigerated. Rewarm soup over medium-low heat before continuing.) Ladle soup into bowls; sprinkle with green onion and serve.