So firstly, cut the mushrooms and onions up, and try to cut the mushrooms smaller for more taste. Don't cut the onions too small though, probably about small pieces will do.
(Do not mix them up together)

Secondly, get a small pot out and place it on the stove, put a small amount of butter on it, and melt. Pour in the onions and stir fry until there is a strong smell. (don't get the onions burnt!)

Then put in the mushrooms and cook both until there is a deliecious aroma. Pour water(usually depending on how much mushrooms and onions you use but for me I used about two cups).

And put in the chicken stock cube, depending on how much you are serving. I used one. Stif for about ten minutes.

Make a mixture using milk and flour, for the thickness.
And pour it in. Stir for fifteen minutes and you can serve (:

Sorry if its confusing, but feel free to ask anything and I'll respond to it as soon as I can! :D