In a big bowl, combine ground beef, diced onion, garlic powder, quick cook oatmeal, shredded cheddar cheese, milk, one egg, salt, and pepper.
Use your hands to mix, but DO NOT OVER MIX. The more mixing you do, the tougher the meat will get.
Lay two large sheets of aluminum foil on the counter in a cross pattern. Place the ball of meat in the center where they cross, and shape the meat into a small loaf.
Fold the foil up so that it creates a "bowl" around the meatloaf, this is where the drippings from the meat will collect.
In a small bowl, combine ketchup, brown sugar, and mustard with a whisk. Spread over the top of the meatloaf.
Place the foil lined meatloaf on an Instant Pot trivet (I use the one that came with my pressure cooker). Add one cup of water to the Instant pot, then lower the trivet into the pot.
Secure the lid and make sure the vent is set to "SEALING".
Pressure Cook on high pressure for 35 minutes.
When cook time ends, allow to naturally release pressure for ten minutes. Do a quick release of the pressure.
Remove meatloaf from the foil (be careful the drippings will be hot). Slice on a platter and serve. You can whip up a little extra of the sauce if you want to add more color (it's not a pretty sight, haha).