Press the Saute function on the Instant Pot and add the olive oil. If using cooked sausage, slice into rounds. If using uncooked sausage, remove from casings and crumble into the pot. Add the chopped onion and cook, breaking up the sausage, until browned and onion is softened. Press Cancel.

Add the bell peppers, water, Italian seasoning, and 1 cup of the marinara sauce. Stir well, scraping up any browned bits from the bottom of the pot to prevent a burn notice.

Add the penne pasta on top and press into an even layer. Do not stir. Pour the remaining marinara sauce evenly over the pasta. Do not stir.

Close the lid and move the steam release valve to Sealing. Pressure cook on High: 2 minutes for pasta with a 6 to 9 minute package cook time; 3 minutes for 10 to 12 minute package time; 4 minutes for 13 or more minute package time. Note: the pot will take about 15 to 20 minutes to pressurize before the cook time begins.

When the cook time ends, carefully quick release the pressure by moving the valve to Venting. Stand back slightly as steam releases fast. Once the float pin drops, open the lid.

Stir the pasta. Any extra liquid will absorb as it rests for a minute or two. Stir in the chopped spinach, then the shredded mozzarella. Serve immediately.