Celeriac and potato soup

I think we are by now all familiar with my obsession with root vegetables.  I'll go underground for just about anything, but suddenly my vegetable box kept turning up some gnarled and gnarly roots: celeriac.  Smooth on one side, the other a mass of little, hair-like roots, the rough exterior hides the pale and delicate flavor on the inside.  Celeriac is fantastic mashed with potatoes, but I like to make it into a creamy soup.  The subtle flavor of celery is set off perfectly by serving it with some thinly sliced baguette spread with a creamy blue like St Agur. 

Because celeriac comes in so many shapes and sizes, and the amount of flesh you can get out of the root side is so variable, I tend to do this recipe by weight.  I also throw in a can of white beans before blending to give it some extra protein and body.  You could leave them out, peel the potatoes, strain the soup through a fine sieve, and serve a silky, light soup.  I personally like the heartier, more rustic version pictured here.   

1 large onion, chopped

1 big clove garlic, minced

1 celery root (about 1.5 lbs.), peeled and cut into 1 inch cubes

2 medium potatoes (about 3/4 to 1 lbs.), cut into 1 inch cubes

4-6 cups vegetable stock depending on how thick or thin you want the soup

1 can white beans 

A good pinch of freshly ground pepper

Parsley

Chop onion and cook it gently in a stock pot.  Add the garlic, and once it's softened, add the celeriac, potatoes, and vegetable stock.  Bring to the boil, then lower heat and let simmer until the vegetables are soft.  Add the beans, then blend until smooth.  Garnish with parsley or blue cheese toasts.

Serves 4.