Preheat the oven to 350 degrees . Place the mushrooms in a 2-quart pie pan. In a medium bowl, combine the beef broth and flour; microwave at low power until slightly thickened, 2 to 3 minutes. Stir in the heavy cream and cayenne; season with salt. Immediately pour the sauce over the mushrooms.
In a medium bowl, mix together the breadcrumbs and parmesan. Sprinkle on top of the mushrooms, top with the butter and bake until browned, about 45 minutes.