Hit SAUTE on Instant Pot, and when it reads Hot, add the olive oil and onion
Cook onion until soft, then add the Italian sausage, and cook until no longer pink
Add the garlic, and cook for 1 minute. Don't allow it to brown
Turn pot off, then deglaze bottom of pot with a little water if needed to remove any browned bits
Add uncooked ziti over sausage
Add water over pasta
Add marinara sauce and diced tomatoes, but DO NOT STIR
Place lid on Instant Pot, make sure valve is set to sealing, and hit PRESSURE COOk for 4 minutes
Pot will take a few minutes to reach pressure, then will cook for 4 minutes. When pot beeps, move valve to venting to do a quick release or gradual quick pressure release by slowly turning valve if too much water is spraying out of pot
When pin drops, carefully remove lid. Stir pasta and sauce together. Add half of the ricotta and 1 cup of mozzarella, and stir
Preheat oven to 350 degrees f
Place half of ziti mixture in a greased casserole dish
Top with rest of ricotta, and stir gently until just blended
Top with remaining mozzarella and dried parsley flakes
Bake in preheated oven for 20 to 25 minutes or until pasta is bubbling and cheese is melted
Allow to sit for 10 minutes before serving