Slice the tops off of the bell peppers. Use a knife and/or your fingers to remove the membranes and any seeds. If the bell peppers won't stand up on their own, you can slice a tiny bit off of the bottom to make them stand.
OPTIONAL Step: Brown the ground beef in your Instant Pot using the Saute function (or in a hot skillet on the stove). You can add the onion to the pan to saute along with the beef, if desired. Crumble the beef as it cooks and turn off the heat once the beef has browned. Transfer the meat to a large bowl and wash out the Instant Pot.
In a large bowl, combine the ground beef (either browned or uncooked, depending on your preference), cooked rice, onion, diced tomatoes, ¼ cup of the tomato sauce, Italian seasoning, garlic powder, salt, pepper and half of the cheese. Stir until everything is mixed together well. Refrigerate the remaining ½ cup cheese to use after the peppers are cooked.
Add 1 cup of water to the bottom of the Instant Pot inner pot (use 1 ½ cups water for an 8 quart pot). Place the metal trivet in the pot.
Spoon the filling into the prepared bell peppers, filling them just to the top. Do not pack the filling in too tightly or it may not cook through. Set the stuffed peppers on the trivet in the Instant Pot. Spoon about 1 tablespoon of tomato sauce on top of each pepper.
Close the Instant Pot lid and turn the steam release valve to the sealing position. If you did NOT brown the ground beef, set the pressure cooking time to 16 minutes at high pressure, using the Pressure Cook or Manual button. If you DID brown the ground beef, set the pressure cooking time to 8 minutes at high pressure. The Instant Pot will take about 15 minutes to reach pressure and then the cook time will start counting down.
When the cook time ends, quick release the pressure by using the handle of a long spoon to carefully turn the steam release valve to the venting position. Wait for all of the steam to escape and then carefully open the Instant Pot lid. Check that the middle of the peppers has reached at least 165° F when measured using an instant read thermometer. If not, you can pressure cook the peppers for a few more minutes.
Top peppers with the remaining cheese. Put the lid back on the Instant Pot for a few minutes to melt the cheese. Serve.