Prepare the Meatloaf Mixture: In a large mixing bowl, thoroughly combine the ground beef, bread crumbs, diced onion, apple, egg, garlic powder, salt, and black pepper. Stir until all ingredients are evenly integrated.

Shape the Loaf: Form the blended beef mixture into a loaf shape. Place the loaf on a large sheet of aluminum foil, folding the foil edges around the meatloaf to form a makeshift loaf pan.

Prepare the Instant Pot: Pour 1/2 cup of water into an Instant Pot, placing the steam rack inside with the handles positioned upward. Carefully set the foil-wrapped meatloaf on top of the rack. Close the lid securely, ensuring the vent is sealed. Set the Instant Pot to high pressure following the manufacturer's guidelines and set a timer for 25 minutes. Allow 10-15 minutes for the pressure to build.

Preheat Broiler: Position an oven rack about 6 inches below the heat source and preheat the broiler.

Release Pressure and Transfer: Once the cooking cycle is complete, perform a quick-release method to depressurize the Instant Pot according to the manufacturer's instructions, which should take approximately 5 minutes. Carefully unlock and open the lid, then transfer the foil-wrapped meatloaf, still on the rack, to a baking sheet.

Prepare the Topping: In a small bowl, mix together the ketchup, mustard, and brown sugar until well combined.

Broil and Finish the Meatloaf: Brush the prepared topping over the meatloaf and place it under the broiler. Broil until the glaze begins to brown, about 5 minutes. For a caramelized finish, broil for an additional 1-2 minutes, if desired.