Start by heating the oil in the inner pot using the sauté setting on high or a small non stick pan, then cook and brown the onions.
When done, turn off the Instant Pot or burner and remove the onions from the pan or pot. Add them to the ground meat along with all of the other ingredients, except for the stock and marinara.
Pour the stock into the bottom of the Instant Pot, set the trivet that came with your Instant Pot in the bottom of the inner pot.
Mix all of the ingredients together thoroughly, and then make uniformly sized balls and set them onto the trivet. Layering the meatballs is perfectly fine, just try not to smoosh the bottom ones by pushing down.
Cover the meatballs with the marinara sauce.
Check your sealing ring and then twist on the locking lid. Set the valve to sealing and select Manual mode for 6 minutes.
When the pressurized cook time has ended, let the pot NPR for an additional 6 minutes before Quick Releasing the rest of the pressure.
Let the meatballs cool for a few minutes before plating.