Peel and cut the potatoes into 1.5-2 inch pieces and add to the bottom of the Instant Pot, then add the water or stock. Add the rack/trivet to the pot over the potatoes.
To make the meatloaf, in a large bowl combine the meatloaf ingredients and mix to combine but do not overmix. Spray a piece of foil with non stick spray, then shape the meatloaf and place on the foil, do not completely cover the meatloaf with foil. Place the foil holding the meatloaf on top of the IP rack.
Secure the lid of the Instant Pot and make sure that the vent is in SEALING position. Select the PRESSURE COOK/MANUAL setting and set the cooking time for 25 minutes at high pressure. The Instant Pot will take around 10 minutes to come to pressure and start cooking. When the cooking program ends, quickly release the steam and remove the lid.
Check the internal temp of the meatloaf, it must reach 160°F/71°C. If it's not ready, Cook for an additional 3-5 minutes at high pressure. Remove from the instant pot and transfer the meatloaf to a baking sheet.
Set your oven to Broil. Prepare the topping by mixing the ingredients in a bowl then brush it on the top of the meatloaf. Place the meatloaf under the broiler for 3-5 minutes or until the top is bubbling and caramelized.
Drain the potatoes in the instant pot, then add butter, milk, garlic if using, salt and pepper. Mash, and adjust the consistency and seasoning to your liking. Serve sliced meatloaf with mashed potatoes.