Heat butter in a non stick pan, add celery and mix. Add onions and saute till soft. Add refined flour and mix and saute for 1 minute or till the raw smells disappear. Take care that the colour of flour does not change.
Add 1 cup vegetable stock and mix well. Add potatoes and mix. Add salt, freshly ground black peppercorns and mix. Add milk and mix well. Cover and cook on medium heat till potatoes are fully done.
Grate processed cheese and add to the pan and mix well. Pour into soup bowl, sprinkle a little freshly ground black peppercorns and serve piping hot.