In a large mixing bowl, combine ricotta, 1 cup shredded mozzarella, shredded Parmesan, minced garlic, egg, chopped basil, chopped spinach, and chopped parsley. Season with salt and pepper and stir until evenly combined.
Transfer the cheese mixture into a pastry bag or a large resealable plastic bag and snip off a corner to create a piping tip.
Leave the jumbo pasta shells uncooked and, using the bag, fill each shell with the cheese mixture. Arrange the filled shells on a plate or cutting board as you work.
Lightly spray the Instant Pot insert with nonstick cooking spray. Place the filled shells into the insert; you may layer them to fit but try to keep them relatively even.
Pour 1 1/2 cups water into the Instant Pot and season the water surface with a little salt and pepper if desired.
Spoon the marinara sauce evenly over the arranged shells. Do not stir.
Lock the lid and set the valve to sealing. Pressure cook on High for 8 minutes.
When the cook time finishes, perform a quick pressure release. Open the lid carefully.
Sprinkle the remaining 1 cup shredded mozzarella over the hot shells and replace the lid (unlocked) or cover loosely for 1–2 minutes to allow the cheese to melt from residual heat.
Serve the stuffed shells warm, optionally garnished with additional chopped basil and extra marinara.
