Prepare the veg.
Discard the top of the leeks (dark green part). Halve and clean the whites, getting rid of any grit. Slice thinly.
Heat oil in a large pan. Add leeks and onions (if using) and cook until softened but not coloured (about 5 min). Stir often to prevent burning.
Add potatoes and celeriac (if using) and continue cooking for another 2-3 min.
Add stock, salt and pepper, and cover.
Simmer until all the veg, especially celeriac is tender (approximately 20-25min).
Remove from heat and puree to desired consistency using hand/immersion blender.
Add creme fraiche, adjust seasoning.
Serve sprinkled with chopped chives.
