We're about to go away for a couple of days - to visit the Tate St Ives amongst other places, and so have been going through the fridge, in order to use up fresh food before we leave.  There was half a celeriac, not sure what to do with it so looked at the River Cottage website and this was what Hugh suggested:

Celeriac Soup

• 50g Butter
• 1 Celeriac, peeled and cubed
• 1 Potato, peeled and cubed
• 1 Leek, trimmed, washed and roughly sliced
• 1 Onion, peeled and roughly chopped
• 1 Garlic clove, sliced
• 1 litre Stock, chicken, vegetable or made from the bones of your Christmas bird
• Parsley & walnut pesto to serve
•  

Amongst the comments underneath were suggestions to serve the soup with:

• toasted hazelnuts, homemade croutons and parsley. 
• fresh sage 
• fresh parsley

I also saw on another site that they'd included some apple to spruce up the taste which sounds worth trying.

The suggestion for vegans was:  

dairy free Pure Sunflower Spread instead of butter

we used Knorr Vegetable Stock Pot 
we also sprinkled in some fresh sage as mentioned above
served with Focaccia and Olive oil/Balsamic vinegar dip.