Celeriac and garlic soup

This is not one of my own recipes, but I cant remember where I picked it up. There is lots of garlic in the soup, so if you arent as fond of it as I am, you can reduce the amount of it, if you like.

Ingredients

1 celeriac
5 floury potatoes
120ml extra virgin olive oil
1 large onion, very finely chopped
1 tsp fennel seeds, ground
10 cloves garlic
Juice of half a lemon
1.5 litres light chicken or vegetable stock (if you are using a stock cube, don't use more than half)
Sea salt and freshly ground white (or black) pepper

Method

1 Peel and then dice the celeriac and potatoes into 2cm pieces and drop them into a bowl of water; add a good squeeze of lemon or a dash of vinegar to keep the vegetables a pure white colour.
2 Heat a large casserole pan over a medium-low heat and add all but two tablespoons of the oil. If you heat the oil too much you will lose all its flavour, so the aim here is gentle cooking. Add the onion and fennel seeds and cook over a low heat until the onion is translucent and melting into the oil, about ten minutes.
3 Meanwhile, bash the garlic cloves to loosen the skin, peel and remove any inner green stems, which have a nasty bitter flavour. Finely chop and add to the pan, cooking for another few minutes. Add the drained celeriac and potatoes. Season well with salt and pepper and cook for another five minutes, turning the vegetables to coat in oil.
4 Cover with the hot stock and simmer for about 20 minutes until a knife pierces the vegetable pieces easily.
5 Add the lemon juice and use a stick blender to blend the soup for only a minute or two. The aim is to have a thick, silky soup texture, but with half the chunks of potatoes and celeriac still intact. Serve with the remaining olive oil drizzled on top and a good grinding of black or white pepper.