Add 3 pounds russet potatoes to the bottom of your Instant Pot. Pour 1 cup water on top.
In a large bowl combine 2 pounds lean ground beef, 1 small onion chopped, 3 tablespoons fresh parsley chopped, ¾ cup Italian bread crumbs, ½ cup milk, 1 tablespoon Worcestershire sauce, 2 large eggs, 1 teaspoon garlic powder, salt and pepper. Use your hands to mix until combined, being careful not to overmix.
Place a sheet of aluminum foil on the counter. Shape the meatloaf into a loaf and shape the sides of the aluminum foil around the meatloaf. Place on top of the potatoes in the instant pot.
In a small bowl, whisk together ¼ cup ketchup, 1 tablespoon balsamic vinegar and 1 tablespoon brown sugar. Brush half of the mixture on the top of the meatloaf.
Cover and turn the Instant Pot to the sealing position. Pressure cook on manual high for 25 minutes. Release the pressure with the quick release. Check the meatloaf to see if it has reached an internal temperature of 155 degrees.
Once the meatloaf is done, remove it from the pan and brush on the other half of the glaze, and place it under the broiler for 5-8 minutes until the glaze is nicely browned.
Add ½ cup milk and ¼ cup butter to the potatoes and mash with a beater or hand masher until they reach the desired consistency, and salt to taste .