I didn't puree this recipe entirely, because I have often noted that I
like texture in my soup. I made a mistake with this one. It should
really be pretty smooth. The chunks made it seem more like baby food
than an elegant soup. The flavour was really nice, though. If you don't
want to take the time to roast the celeriac first, you can put it in
raw at the same time as the stock and the potato. I didn't have any
leeks, so I just used a large onion.