I always use cornstarch when I am making a white sauce. I always just guess on how much to use. (I'm a dumper) Most of the time when I want some kind of grave though I just stick with Progresso Cream of Mushroom soup and some beef broth then I season it up for what ever kind of dish I'm making.

I like to make mini meatloaves and fry them, then I deglaze the pan with some beef broth add the mushroom soup, and season it with what ever you please. Then serve with potatoes or brown rice.