Preheat The Oven To 350 Degrees F (175 Degrees C). Crumble The Ground Beef Into A Large Cast-Iron Skillet Over Medium-High Heat. Cook, Stirring Frequently, Until Beef Is Evenly Brown. Drain The Grease, And Add The Onion. Cook And Stir Until The Onion Is Translucent. Add The Red Peppers, Beans, And Fajita Seasoning; Cook And Stir Until Heated Through. Spread Out In An Even Layer On The Bottom Of The Skillet. Mix The Package Of Cornbread Mix According To The Directions Using The Egg And Milk. Spoon Over The Ground Beef Mixture, And Spread Evenly. Place The Whole Skillet In The Oven, And Bake For 20 Minutes, Or Until A Toothpick Inserted Into The Cornbread Layer Comes Out Clean. Cool For A Few Minutes Before Serving.