Turn the Instant Pot on saute mode. When hot, add the sausage and cook, stirring often until no longer pink. Remove from the pot to a large bowl. Deglaze the inner pot to remove any pieces stuck to the bottom of the inner liner.
In the large bowl of browned sausage, add the ricotta cheese, mozzarella cheese and next 4 ingredients. Stir until well combined and then stuff into the jumbo shells. Place shells in a casserole dish that fits inside the inner liner of the Instant Pot.
Mix together the diced tomatoes, tomato sauce, tomato paste and Italian seasoning. Pour over the stuffed shells. If the shells are not completely covered, add just enough water until they are all covered.
Place the trivet in the bottom of the inner liner and add 1 ½ cups of water. Lower casserole dish onto trivet.
Close the lid of the Instant Pot and turn the pressure valve to sealing. Cook on manual high pressure for15 minutes, then quick pressure release when the cooking time has completed.
Carefully remove dish from inner liner using the handles of the trivet. Serve warm with salad and Italian bread. Top with grated parmesan, if desired.