In a food processor, or by hand, chop mushrooms into fine pieces. It should be the same size/consistency as ground beef.
In a large skillet, brown ground beef in 2 tablespoons butter; drain. Add mushrooms, onions, carrots, 2 tablespoons of butter, beef broth, Worcestershire sauce, minced onion, ground mustard, and tomato paste to skillet; cook for 10 minutes on medium heat. Season with salt and pepper to taste.
Place beef mixture in the bottom of a deep casserole dish; set aside.
In a clean skillet, sauté cabbage in remaining butter, garlic power, onion powder; season with desired amount of salt and pepper. Add mashed potatoes to skillet and stir until combined.
Spread cabbage mixture on top of beef in casserole dish; bake at 350° for 15 minutes.
Broil casserole until it just begins to brown on top (approx. ten minutes)
