Turn the instant pot to saute mode and add the olive oil.
Once the oil is hot, add in the sausage and onion and cook, breaking up the meat as you go until cooked through, about 5-6 minutes.
Turn the pot off and add in the minced garlic, oregano, basil, red pepper flakes, and salt & pepper to taste. Stir constantly for 1 minute.
Add the chopped sun dried tomatoes, broth, water, and pasta shells to the pot.
Place the lid on the pot and set the valve to sealing position. Cook on manual, high pressure for 4 minutes, then do a quick release.
Remove the lid and add the cream and spinach to the pot and stir constantly just until spinach is wilted.
Add the cheese to the pot in 3-4 handfuls, stirring well between each addition. Serve immediately. Leftovers should be stored in an airtight container in the refrigerator and eaten within 3 days.
Notes
NOTE: It is important to make sure that you add the cheese in 3-4 additions, stirring well between each one. If you add it all at once, it may clump together.