In a medium bowl, mix together spinach, ricotta and mozzarella, garlic, Italian seasoning and salt & pepper.
Spoon the spinach-cheese mixture into each pasta shell. Alternatively, line a large tall glass bowl with zip lock bag. Gently pour the spinach/cheese mixture into the bowl lined with plastic zippered bag, zip close, and cut the tip off of one corner for easy squeezing into the shells. Squeeze the mixture into each shell.
Press the SAUTE button on your Instant Pot and add oil. Wait few seconds then add sliced mushrooms and stir for few minutes till mushrooms start to brown and release water. Cancel SAUTE.
Add water to the pot. Make sure to deglaze the bottom of pot by scrapping the base with wooden spatula to ensure nothing is stuck at the bottom.
Layer the stuffed shells by keeping them upright. If you have more than one layer, arrange on top of each other . Ensure that they stay upright. Do NOT put more than 2 layers at once.
Pour the jar of marinara sauce over the stuffed shells. Do NOT Stir.
Lock the lid on the Instant Pot.
Press MANUAL and adjust the time to 6 minutes and close the steam valve.
Once IP is done cooking, wait for 5 minutes . Then release the steam valve to venting to release the steam and pressure. Open the lid carefully.
Sprinkle parmesan cheese over top and place the lid back on for 1-2 minutes or until cheese melts.
Scoop out shell with sautéed mushrooms in a dinner plate. Serve warm with optional ground black pepper, chili flakes and parmesan cheese sprinkled on top. Enjoy!