First peel and chop the onions finely. In a thick bottomed cooking container, add butter and the chopped onions. Fry the onion till it becomes shiny. (Do not to brown it). Add water along with a pinch of salt and pepper powder. Allow to boil for five minutes. Or you can pressure cook it for one whistle. When it is cooled down, put it in a liquidizer and make a fine puree. Pass through a sieve.
Put this back in a vessel and allow to boil again. When it starts boiling, reduce the heat. Mix the flour in the milk and add it to the soup. Stir continuously for few more seconds and remove.
Garnish with chopped coriander leaves. Add salt and pepper powder and serve hot.
If you wish, you can add little fresh cream while serving.