Season the chicken with salt, pepper, and Italian seasoning.
Heat the Instant Pot using the SAUTE function, then add the olive oil. Add the seasoned chicken to the hot oil and do not move it around for the first 2 minutes so it can sear, then continue to saute, stirring occasionally for another 1-2 minutes until the chicken is almost cooked through.
Add onions and cook until chicken is done and the onions have softened slightly.
Add chicken broth and pasta, using a wooden spoon to scrape the bottom of the Instant Pot to remove any bits of chicken or onion.
Place the lid on the Instant Pot, setting the valve to sealing and switch the Instant Pot to the PRESSURE function. Cook on HIGH for 5 minutes, followed by a quick pressure release.
Remove the lid and stir in butter, heavy cream, spaghetti sauce, and mozzarella cheese until everything is melted and hot. If needed, switch back to the SAUTE feature.
Sprinkle with parmesan cheese and fresh parsley before serving.