Melt butter in pan and sweat onions till starting to brown. Add garlic and cook till fragrant. Add mushrooms and season. Cook till all liquid has evaporated and mushroom have browned.

Add 2nd butter and melt. Stir in flour and cook roux for several minutes or until roux is light brown.

Add remaining ingredients stirring to dissolve roux in stock. Adjust seasonings. Bring to a boil and then reduce heat. Simmer until thickened.

NOTE:
To intensify beef flavor, I added a splash of Minor's Au Jus liquid which worked very well.

NOTE 2:
For another layer of flavor you could add some cognac to the onions before you add the mushrooms and burn off the alcohol. Other wines/liquors would work as well.

Note 3:
If you want more of a gravy, add some heavy cream at the end.