Heat the oil in a large pan and fry the chopped onion and garlic until soft. While the onions are cooking peel and chop the celeriac and potatoes. Next add the bay leaves, sprigs of thyme, celeriac and potatoes. Pour over the stock and bring to the boil. Then place on a lid and simmer for 20 minutes.
To make the Brie and Cranberry Croutons
These are best prepared fresh - so if preparing soup in advance don’t make the croutons until you are ready for them. Place a handful of fresh cranberries into a small container or cup and cover with boiling water - set to one side for 2 minutes. Next slice 1 cm thick circles from a French stick. Each circle will be a crouton for the soup - prepare as many croutons as you need. Place the bread rounds on a baking tray and top each one with a slice of brie. Next carefully remove the cranberries from the hot water and push one or two cranberries into the cheese on each piece of bread. Heat the grill.
Finishing the soup and serving
After 20 minutes remove the lid from the soup pan. Find and remove the bay leaves and thyme. Use a hand or stand blender to blitz the soup until smooth. To make it extra smooth pass well blended soup through a sieve.
Finally, season and stir in the honey and creme fraiche.
To serve, reheat the soup and place the croutons under a high grill for 5 minutes or until the cheese is melted. Ladle out the soup, top with hazelnuts and truffle oil and place a grilled crouton on the top.
Or alternatively store the soup in sealable containers and keep in the fridge for up to 3 days or the freezer for up to 3 months.