Melt the butter in a lidded saucepan and fry the bacon until golden-brown. Remove from the pan using a slotted spoon and set aside.
Fry the onion in the remaining fat until soft. Rinse the potatoes under cold running water until the water runs clear. Drain the potatoes. Pour the stock into the pan, and then add the potatoes, salt and freshly ground white pepper and bay leaf.
Bring up to a gentle simmer, skimming off any scum that forms on the surface, and add the bacon. Cover with a lid and simmer for 25-30 minutes, or until the potatoes begin to break up. Remove the bay leaf and bacon (you can crisp the bacon up for a sandwich if you like).
Pass the mixture through a vegetable mill using the finest mesh disk into a clean pan. You can also blend the soup in a food processor - though I find this method produce too smooth a result.
Gently reheat and ladle into hot bowls. Serve with a good dollop of soured cream and sprinkle with plenty of chives.
Make this a vegetarian soup by using vegetable stock instead of chicken stock and leave out the bacon.
