Heat olive oil in a large pot over medium heat.
Add the onion and sauté for 3–4 minutes until soft and fragrant.
Stir in garlic and cook for another 30 seconds.
Add chopped carrots and parsnips, stirring to coat them in the aromatics.
Pour in the vegetable broth and water, then add salt, pepper, and thyme.
Bring to a boil, then reduce heat and simmer for 20–25 minutes until vegetables are soft.
Turn off the heat and blend the soup using an immersion blender or regular blender until creamy and smooth.
Add cream or butter if using, and adjust seasoning to taste.
Serve warm with fresh parsley, a drizzle of cream, or croutons.