Chop mushrooms super fine or blend in a food processor. In a larger bowl, combine ground susage and mushrooms and kneed until mixed well.
Turn Instant Pot onto saute and wait until it is hot. Add meat mixture, salt, pepper, and garlic. Cook meat until no longer pink then drain off excess fat and water. Turn Instant Pot off.
Add green pepper, onion, black olives (reserving a few for the top), pepperoni (reserving a little for the top), rotini pasta, pizza sauce and water. Do not stir.
Close Instant Pot and set to sealing position. Select manual high pressure (the manual button or the pressure cook button on the newer models) and set timer for 5 minutes. When time is up, perform a quick release.
Give everything a good stir then top with mozzarella, remaining black olives, and pepperoni. Close lid and allow to sit for 5 minutes to melt the cheese.
Serve with shredded Parmesan cheese and crushed red pepper flakes for a fun spin on traditional pizza!