The first step is to turn the Instant Pot onto "Saute" and allow it to heat up.
Once hot, add in the onions and olive oil. Cook this until the onions soften up.
Add the ground beef and stir so that it does not stick to the pot. Make sure the beef is cooked until it is brown.
Add in the salt, pepper, garlic powder, and Italian seasoning.
Hit "Cancel" on the Instant Pot. Add in the shells and marinara sauce. Put in the water but do not stir this together. Press it with a spoon so all of the water and sauce is covered in the pasta.
Seal the vent on the Instant Pot and close the lid.
Hit "Pressure Cook" for 5 minutes. After that is done, quick release the pressure. Remove the lid, and hit "Cancel" on the Instant Pot.
Stir the heavy cream in there and add the parmesan cheese.