In a bowl, mix spinach, ricotta, 1 cup mozzarella, parmesan cheese, whisked egg, salt, pepper and minced basil.
Stuff your pasta shells with the prepared filling.
Pour olive oil into your Instant Pot and add onion and garlic.
Press “Sauté” and cook for 2 minutes, until onion is golden.
Add marinara sauce, stir and cook for 1 more minute.
Add water and stir well.
Place your stuffed shells into the sauce and form a compact layer.
Cook at high pressure for about 8 minutes.
Quickly release the steam and open the lid.
Sprinkle with the rest of your mozzarella and place the lid back on (the heat will make mozzarella melt in a couple of minutes).