Mix Ingredients: Grate the onion into a large mixing bowl. Stir in the breadcrumbs and milk and let the mixture sit for 2 minutes. Add the ground beef and pork, egg, Worcestershire, salt, and pepper. Mix everything well to combine.
Form the Loaf: Shape the meat mixture into a loaf shape that will fit in your Instant Pot, round works best. Place the loaf onto a large sheet of foil, and fold the edges up to create a nest around the loaf.
Prep Instant Pot: Pour the 1/2 cup of water into the Instant Pot. Place your Instant Pot's rack inside the pot, then lift the foil carrying the loaf and place it on top of the rack.
Cook the Meatloaf & Make the Topping: Secure the lid of your pot. Turn the valve from VENT to SEALand cook on high pressure for 25 minutes. Meanwhile, mix the ketchup, brown sugar, mustard, and Worcestershire sauce together for the topping.
Top with Sauce: Turn your oven's broiler on. Quick release the pressure on the instant pot and, using two tongs or gloved hands, carefully lift the foil and loaf out of the pot and place it on a baking sheet. Brush the top of the loaf generously with the prepared sauce.
Broil & Serve: Broil for 2-5 minutes, or until browned and bubbly. Garnish with parsley and serve.