Heat oil or schmaltz in a pot over medium heat.

Add onion and celery and sauté slowly until very soft and lightly golden. This step builds the soup’s flavor, so take your time.

Add parsnips and potato and stir to coat with the softened vegetables.

Pour in stock or water, just enough to cover the vegetables.

Bring to a boil, then reduce to a gentle simmer. Cook until all the vegetables are very tender, about 25–30 minutes.

Blend until completely smooth using an immersion blender or in batches.

Season with salt and freshly ground black pepper. Taste and adjust as needed.

For a thicker, velvety potage, keep the soup dense. For a lighter first course, thin with a bit of additional stock after blending.