In a soup pot, melt the butter and cook the bacon for a few minutes without browning. Add the leek, onion, celery and bay leaf. Sweat for about 10 minutes without browning.
Add the celeriac and sweat another 5 minutes. Add the stock and potatoes and bring to a full boil. Reduce the heat, cover and simmer about 30 minutes. Remove the bay leaf.
Blend the soup until smooth and strain to remove the fibres. Season to taste and adjust the consistency with a little milk, if necessary.
Just before serving, reheat the soup on the stovetop and when it is very hot, add the cheese, whisking until it melts. Serve immediately, garnished with croutons and celery leaves.
