In a large saucepan, heat the olive oil over medium heat. Add the onion, leeks, garlic, celery, parsnips, and ginger.
Cook until soft—about 5–10 minutes.
Add the stock and lemon.
Bring to a boil, then reduce to a simmer.
Partially cover, then cook for twenty minutes.
Remove from heat, then remove the lemon.
Purée the soup in batches in a blender, or use an immersion blender to purée.
Stir in the yogurt, pepper, and Parmesan.
Garnish with chives, then serve.