Cut the onion, celery and carrot into a 1cm (½") dice.
Melt the butter in a 24cm or 10" saucepan over a medium-low heat and add the onion, celery, carrot, fennel seeds and black pepper and "sweat" for 10 minutes. If the onion begins to brown, turn down the heat.
Peel the celeriac and potatoes and cut both into 25mm (1") dice.
See the Frequently Asked Questions for tips on how to prepare celeriac.
Pour in the vegetable stock and add the potato, celeriac, bay leaf and salt and bring to a simmer.Add a lid and cook for 30 minutes or until the potato and celeriac are cooked.
Transfer to a blender and blitz to form a smooth liquid.
Return the soup to the pan that it was cooked in passing it through a fine-mesh sieve.
Now add the cream and milk, stir to combine and bring to serving temperature, now is a good time to check the seasoning again and add salt as required.