** Unsalted canned chicken broth may be used instead of stock. *** Green apples should be cored, peeled and chopped (about 2 cups) Place squash cut side down on rack set over gently simmering water in saucepan. Cover and steam until tender, about 10 minutes. Cool squash slightly. Scoop pulp from shells.
Combine 1/4 cup chicken stock, apples and onion in heavy medium saucepan. Cover and cook over low heat 10 minutes. Add squash pulp, remaining chicken stock, apple juice, ginger and salt. Cover and simmer until ingredients are very tender, about 20 minutes.
Puree soup in batches in processor or blender. Strain through sieve into clean saucepan, pressing puree with back of spoon. Reheat soup gently. Add lemon juice. Season with salt and generous amount of pepper. Ladle into bowls. Garnish with yogurt and chives.