Celeriac, potato and mustard soup

Melt the butter in a saucepan, add the onion and leek, cover and cook on a low heat for 5 minutes until soft. Add the celeriac and potatoes and cook for a further two minutes. Pour on the chicken stock and bring to the boil, reduce the heat and simmer until the vegetables are tender. Transfer to a blender and blitz until smooth. Reheat the soup to the boil, whisk in the mustard, cream and chives. Season to taste and serve.