In a large mixing bowl, combine the 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 mixed.
Transfer the cheese mixture to a pastry bag or a resealable plastic bag and snip a corner to make a piping tip.
Do not cook the shells. Pipe the cheese mixture into the uncooked jumbo shells and place the filled shells on a plate or cutting board as you work.
Spray the Instant Pot insert with nonstick cooking spray. Add 1 1/2 cups water to the insert and season the water lightly with salt and pepper.
Arrange the filled shells into the Instant Pot insert. It is fine to layer the shells to fit them all.
Pour the marinara sauce over the arranged shells without stirring.
Lock the lid, set the valve to sealing, and pressure cook on High for 8 minutes.
When the cook time ends, perform a quick pressure release according to your Instant Pot's instructions, then carefully remove the lid.
Sprinkle the remaining 1 cup shredded mozzarella over the shells, close the lid (not sealed) or cover loosely, and let sit 1–2 minutes until the cheese melts from the residual heat.
Serve the stuffed shells garnished with additional chopped fresh basil and extra marinara if desired.