In a large pot, melt the butter over medium heat. Add the chopped onion and sauté until translucent.

Add the diced parsnips, potatoes, and carrots. Cook for about 10 minutes, stirring occasionally.

Pour in the vegetable broth, ensuring the vegetables are covered. Bring to a boil, then reduce heat and simmer for 20 minutes until tender.

Blend the mixture until smooth using an immersion blender. Adjust seasoning with salt and pepper. Stir in crème fraîche if desired.