In a large mixing bowl, combine ground beef, salt, black pepper, garlic powder, and onion powder. Mix until well combined.
Divide the beef mixture into four equal portions and shape each into a patty, about ¾ inch thick.
Heat a large skillet over medium heat. Sauté chopped onions and mushrooms for about 5 minutes until soft. Push to the side and add beef patties, cooking for about 4 minutes on each side until browned.
In a greased 9x13-inch baking dish, layer the beef mixture, sautéed onions, and mushrooms. Pour in beef broth, Worcestershire sauce, and soy sauce. Stir gently to combine. Sprinkle frozen mixed vegetables on top.
Spoon prepared mashed potatoes over the mixture, spreading evenly. Sprinkle shredded cheddar cheese on top. Bake in a preheated oven at 375°F for 25-30 minutes until cheese is bubbly and golden.
Let cool for about 5 minutes before serving.