Gather all your ingredients and pre-measure things before you start. Once you melt the butter in the first step, it all goes together very quickly, so be prepared with all your ingredients "ready to pour"!
In a large saucepot, melt the butter. Saute' the onions for about 8 - 10 minutes, until they are a lovely golden brown.
Add the sliced mushrooms, and stir and cook until mushrooms are tender and 'wilted'.
Sprinkle the flour over the top, stirring continuously. It will tend to "seize up" on you once you add the flour, but continue to stir well and cook for about 2 minutes, until bubbly and golden.
Quickly whisk in the chicken broth, stirring vigorously to break up flour lumps.
Whisk in the milk, stirring constantly, until mixture thickens. [If, at this point, you decide you'd prefer it a little thicker, you can add a water/cornstarch mixture to thicken it to your liking].
Add the dry mustard, salt and pepper, and Worcestershire sauce, stirring well to blend.
Add sour cream, stir well to blend. Cook on LOW for about 3-5 minutes to allow flavors to blend. Do not allow to boil or scorch!
You can put this thru a blender to break up the mushrooms, if you prefer.
Leave out the center hole of your blender lid, in order to allow steam to escape. Blend in small batches. Return to stove to reheat. [I prefer to puree only 1/2 of the mixture, and add it back to the pot, so there will be some mushrooms left for 'texture'.]
Sprinkle parsley on top when serving. Serve with a fresh, chewy baguette, and a fresh salad.