1. Heat the butter in a deep non-stick pan, add the onions and mushrooms and sauté on a medium flame for 1 minute.
2. Add 3 cups of hot water, mix well and cook on a medium flame for 5 to 6 minutes or till the mushrooms get cooked.
3. Allow it to cool slightly and blend in a mixer to a smooth transfer the purée into the deep non-stick pan, add the salt, pepper and milk, mix well and bring to a boil.