Combine the meatloaf ingredients in a large bowl, mixing until incorporated but taking care not to overmix the meat. Shape into a round loaf that will fit inside the Instant Pot.

Arrange the quartered potatoes in the bottom of the Instant Pot, then set the wire rack over them. Add the chicken broth to the potatoes.

Create an aluminum foil "pan" for the meatloaf by layering two sheets of foil on top of each other, then placing the meatloaf mixture on the center of the foil and wrapping the foil up around the sides to create a pan that is tall enough to catch the fat and juices from the meatloaf as it cooks.

Set the meatloaf in its wrapper on the wire rack over the potatoes. Place the Instant Pot lid on and turn to seal, then cook on high pressure for 30 minutes, followed by a 10 minute natural release. Do a quick release by turning the steam release knob to the vent position to release any remaining pressure. Check the meatloaf with a digitial meat thermometer inserted into the center to make sure it has reached 160 degrees F. If not, put the lid back on the Instant Pot and cook for another 10 minutes.

Suction out the drippings from around the meatloaf using a baster and discard, then carefully remove the meatloaf in it's foil pan from the Instant Pot and transfer to a baking sheet. Combine the sauce ingredients in a bowl, then spread over the top of the meatloaf. Broil for 2-3 minutes until caramelized on top.

Finish the potatoes by adding the butter, milk (or half & half), sour cream, and salt, and mashing to your desired consistency. Taste and adjust salt, if needed.