*Use all of the white part and about 2 inches of the green part or, in the case of leeks, as much of the green as is tender; don't use the tough parts.
Melt the butter in a large saucepan, and saut? the spring onions or leeks, onions, and mushrooms until the onions are translucent and the liquid from the mushrooms has mostly evaporated.
Stir in the flour, and cook gently for several minutes. Stir in the chicken broth or water a little at a time; if you keep stirring, it won't become lumpy. Add the milk, and bring to a boil. The liquid will thicken just a bit. Add salt and pepper to taste