Put the stock, potatoes, leeks, celery, bay leaf and thyme in a large
pot and sprinkle with salt and pepper. Boil until the potatoes are
soft, 15 to 20 minutes.

Remove the bay leaf. Using an immersion blender (or in batches in a
blender or food processor), blend the soup until smooth. Pour the soup
into a medium pot; add the cream and buttermilk, simmer until the soup
has thickened, about 20 minutes.