Step 1

In a large pot, melt the butter with the olive oil over medium heat.

Step 2

Add the leeks and cook for about five minutes, stirring frequently. Add the garlic cloves and season with salt, and continue to cook until the leeks and garlic are soft and translucent. If the leeks begins to stick and brown too much on the bottom, add a pat of butter or pour of olive oil.

Step 3

Add the cubed celery root and water. Increase the heat to high, bring to a boil, then reduce to a strong simmer. Add the bay leaf and thyme, if using, and cook with a lid ajar on top until the celery root pieces are soft and easily pierced with a paring knife, about 30 to 45 minutes.

Step 4

Turn off the heat and add white pepper and chili powder, pluck out the herbs, if you used them, then purée the mixture using an immersion blender, or let the soup cool to room temperature and whiz in a standard blender until smooth. (Do not puree hot soup in a blender as it can create a vortex and blow off the lid, which is dangerous.) Taste the soup, and season with additional salt and pepper if desired. If the soup is too thick, it can be thinned with additional water.

Step 5

To serve, rewarm the soup in a saucepan and ladle into bowls.