Peel and finely chop the potatoes and onion.
Melt butter in a pan and sauté the onion.
Add the potatoes and cook together.
Pour in chicken stock and simmer until the potatoes are tender.
Blend the potatoes until smooth using a blender.
Return the soup to the pot and add milk, then bring to a simmer.
Season with salt and pepper to taste.
Serve in a bowl garnished with parsley or croutons.