Cook Onion and Beef:
Pour the olive oil into the Instant Pot and select the Sauté setting. Add the chopped onion and cook until softened, about 3–5 minutes. Add the ground beef, Italian seasoning, garlic powder, minced garlic, and salt and pepper. Cook while breaking up the meat until fully browned. Drain excess grease, then remove the meat mixture from the pot and set aside.
Arrange the Spaghetti:
Turn off the Sauté function. Add 1½ cups of water to the bottom of the Instant Pot. Break the spaghetti noodles in half and place them in a crisscross pattern to reduce sticking.
Add Liquids and Sauce:
Pour the pasta sauce, diced tomatoes, and remaining 1½ cups of water over the spaghetti. Make sure the noodles are completely covered by the liquid.
Pressure Cook:
Secure the lid on the Instant Pot and set the valve to Sealing. Cook on High Pressure using the Manual or Pressure Cook setting for 5 minutes.
Release Pressure:
After cooking, let the pressure release naturally for 5 minutes. Then carefully switch the valve to Venting to release any remaining pressure.
Mix and Serve:
Once pressure is fully released, open the lid and return the cooked meat mixture to the pot. Stir well until everything is evenly combined, then serve and enjoy.
