Heat a pan over medium and melt the butter. Fry beef and onions for a few minutes until the onions are softened, then add the broccoli, mushrooms, soy sauce and hot sauce. When the broccoli is softened, turn up the heat and add your flour and water mixture. Stir briskly for about 30 seconds, and remove from heat. Stir in sour cream and let settle two minutes before serving alone, over rice, or over egg noodles.