Turn the Instant Pot to saute. Cook ground beef and onion, stirring often, until beef is browned and onion is translucent, about 4 minutes. (If using regular ground beef instead of lean, you may want to drain remaining fat before continuing)
Add Italian seasoning, salt and pepper and cook 1 minute.
Add water and scrape the bottom of the pressure cooker with a wooden spoon to remove any bits that might have stuck on the bottom. (*You risk getting the burn message if there is anything stuck on the bottom of the pot!)
Turn the Instant Pot off and add the macaroni. Do not stir, simply sprinkle on top of the ground beef in an even layer. Press down lightly.
Cover the macaroni with the diced tomatoes and pasta sauce, doing your best to cover all of the pasta.
Put the lid on, turn the valve to sealing, and select Manual or Pressure Cook, high pressure, and set the cook time to 2 minutes. It will take about 10-15 minutes to build pressure and begin counting down.
When the cook time is up, turn the Instant Pot off and open the valve to release the pressure.
It will look like a lot of liquid on the top, but once you stir the sauce will coat the pasta and it will not be watery. Serve.