I do my mushroom sauce a little different. Melt two tablespoons of butter in a saucepan, and dump in a cup of mushrooms. Add salt, freshly ground, coarse black pepper and a dash of garlic (if it's your thing). Let the mushrooms slooowly fry until the pan is fairly dry. Add another tablespoon of butter, and add a tablespoon of flour, and mix well. Add a cup of cream, and slowly bring to the boil. Add a little bit more flour depending on how thick you want the sauce. After it's thickened you can adjust the salt and whatnot if you wish.
This is the only way I can make a mushroom sauce so it adapts to that classic grey colour, and it's awesome. I sometimes also make that sauce, but 3 times as much with less flour, then I'll drop some simmered fillet medallions into the sauce and let them float-cook on top of the sauce until done. A.MA.ZING.
Should make it again. Soon, methinks.