Turn on a multi-functional pressure cooker (such as Instant Pot) and select Saute function. Add oil and allow to heat. Add onion and peppers to hot oil; cook until they begin to get tender, 3 to 4 minutes. Add sausages and cook, breaking them up with a wooden spoon, until lightly browned, 4 to 5 minutes. Add garlic and saute for 1 to 2 minutes. Add marinara sauce, water, spinach, pasta, basil, and Italian seasoning; mix to combine. Cancel Saute function.

Close and lock the lid. Select high pressure according to manufacturer’s instructions; set timer for 5 minutes. Allow 10 to 15 minutes for pressure to build.

Release pressure using the natural-release method according to manufacturer’s instructions, 5 minutes. Then release pressure carefully using the quick-release method according to manufacturer’s instructions, about 5 minutes. Unlock and remove the lid. Stir in 1/2 cup shredded Italian cheese until combined. Serve with remaining cheese sprinkled on top.