Melt butter in a lidded saucepan and fry bacon until golden-brown.
Remove bacon with a slotted spoon and set aside.
Fry chopped onion in the remaining fat until soft.
Rinse 500g of potatoes under cold water and drain well.
Pour 750ml of chicken stock into the pan with potatoes, salt, pepper, and bay leaf.
Simmer gently while skimming the surface and return the bacon to the pan.
Cover and cook for 25 to 30 minutes until potatoes break up.
Remove the bay leaf and bacon from the soup.
Pass the mixture through a fine vegetable mill or blend for desired texture.
Reheat the soup and serve with soured cream and fresh chives.