ok, for the mushroom sauce I have a fail proof super dooper easy one.
I slice up about a dozen mushrooms (more or less depending on how 'mushroomy' you want it) and throw them into a hot pan with butter. I then add about 2 teaspoons of whole grain mustard (you could replace with Worcestershire). Fry off until mushrooms are soft and starting to caramelise, and add about 200ml of thickened cream. stir until simmering and serve.
This is an awesome basic recipe. I have used field mushrooms, cream cheese, no mustard, added cheese... really the ideas are endless. I find the older cup mushrooms that are open make a much nicer sauce. I am an Aussie, so I dont know what the conversion is to ounces, but its about 1 small tub of cream - but really, I go more by look than measurements.
Steak, mushroom sauce, mashed potatoes and green salad sounds awesome to me. Maybe some grilled zucchini or asparagus to add a bit more green?