- 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 ¼ 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.