Set the Instant Pot to sauté mode and cook the ground beef until browned. Drain any excess fat.
Add the diced onion and minced garlic, cooking for another 2 minutes until fragrant.
Turn off sauté mode and stir in the spaghetti sauce, diced tomatoes, water, salt, pepper, and Italian seasoning.
Break the spaghetti in half and layer it in the pot, pressing it down gently without stirring.
Close the lid, set the Instant Pot to manual high pressure for 8 minutes, and do a quick release when done.
Stir the spaghetti to mix everything well, then serve hot with grated Parmesan cheese on top.