Turn on saute mode on the Instant Pot and add the olive oil to the pot.
If using sausage that is already cooked, slice it and add it to the Instant Pot. If using uncooked sausage, remove it from the casings and add it to the pot.
Add the onion to the pot and saute the onion and sausage for 2-3 minutes, or until sausage is browned (for uncooked sausage, break it up into smaller pieces as it cooks). Press cancel to turn off saute mode.
Add the bell peppers, water, Italian seasoning and 1 cup of the marinara sauce. Stir, scraping up any bits from the bottom of the pot.
Add the penne pasta to the pot and do not stir. Press the pasta down into an even layer. Pour the remaining sauce evenly over the top of the pasta; do not stir.
Close the Instant Pot lid and turn the steam release valve to the sealing position. Set the cook time to 2 minutes at high pressure. (This 2 minute cook time is for whole wheat penne pasta with a stovetop package cook time of 9 minutes for al dente. You can adjust your pasta cook time as needed, see my tips in the post above.)
The Instant Pot will take about 18 minutes to reach pressure and then the cook time will begin counting down. As soon as the cook time ends, do a controlled quick release of the pressure by turning the steam release valve towards the venting position, a little bit at a time. I like to use the handle of a long spoon to carefully move the valve. When all of the steam has escaped and the pin drops down, carefully open the Instant Pot lid. You may see a little bit of extra liquid in the pasta, but this will be absorbed as the pasta rests.
Stir the pasta and then stir in the spinach, followed by the shredded mozzarella cheese. Serve.