1. In a large pot, melt the butter with the olive oil.
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 pot begins to brown too much on the bottom as they cook, add another pat of butter or pour of olive oil.
3. Add the celery root and stock. (Or use all water.) Bring to a boil, then reduce to a strong simmer. Cook, with the lid to the pot ajar on top, until the celery root pieces are soft and easily pierced with a paring knife, about forty five minutes.
4. Add white pepper and chile powder, the purée using a hand mixer, or let the soup cool to room temperature and whiz in a blender until smooth. Taste, and season with additional salt and pepper if desired. If the soup is too thick, it can be thinned with water or stock.
To serve, rewarm the soup in a saucepan and ladle into bowls.