Melt the butter in a large nonstick skillet over medium heat. Add the mushrooms and cook for 8-10 minutes or until tender. Stir in the flour and cook for an additional 2 minutes until golden. Whisk in the beef broth and heavy cream. Continue to whisk until the sauce begins to thicken and there are no flour clumps remaining. Stir in the parsley, thyme, and salt and black pepper to taste. Bring to a simmer and cook for an additional 5 minutes (more time as needed) until extremely thick. Reduce the heat to low, cover and keep the mushrooms warm while you prepare your steak.