Turn the pot to SAUTE. Add olive oil to the pot, then add the spinach. Saute until it wilts all the way down. Season with salt and pepper to taste. Then pour the spinach into a strainer to remove any extra liquid. Set aside.
Turn the pot off of SAUTE. Add a splash of broth to the pot and use a wooden spoon to scrape up any bits of cooked spinach stuck to the bottom of the pot.
In a bowl, make the filling by mixing ricotta, 1 cup mozzarella, parmesan, garlic, egg, salt, pepper, and spinach. Use a spoon to scoop into the uncooked shells.
Once all shells are filled, spray the liner of the Instant Pot with nonstick spray. Add the broth into the pot. Then add the stuffed shells, stacking on top of one another if necessary.PLEASE NOTE: Some newer brands of the Instant Pot have a more sensitive burn notice and it doesn't always like cooking the noodles on the bottom of the pot. You can also place the shells on the trivet and fill the pot with enough water or broth to cover the bottom layer of shells halfway. Still top with marinara sauce. With this method, you may need a bit more sauce at the end to spoon over the hot shells.
Pour the marinara marinara sauce over all of the shells. If you need more marinara sauce to cover the shells completely, you can use more. Any shells not covered by liquid will not cook correctly and will come out hard.
Put the lid on the Instant Pot and set the pressure to HIGH for 9 minutes. After the timer goes off, allow for a 9 minute natural pressure release.
Open the lid and sprinkle the remaining 1 cup of mozzarella cheese on top of the shells. Put the lid back on (don’t turn the pot on) and allow the residual heat to melt the cheese for about 3 minutes. Open the lid and serve. Enjoy!