I read this on one of the health magazines and tried it out and it tasted so good.
1) Cut the mushroom and take a few pieces of mushroom fry and keep aside.
2) Cut the onions and fry them in little oil and add the remanining mushroom to it. Fry them for sometime and then add the cornflour powder to it.
3) Once the onion and the mushroom are fried well, grind to a smooth paste.
4) Add the butter in a pan and add the ground paste to it and add the milk to it and let it boil for few mins.
5) Then add little salt and pepper to taste and serve hot.
I have tried the same recipe for tomato soup and it came out very well too.