Press the "saute" button on the Instant Pot and add the oil. After it heats up, add in the shrimp. Sprinkle the shrimp with 2 tsp of the cajun seasoning. Cook for 1-2 minutes on each side until cooked through and no longer opaque. Remove the shrimp using a slotted spoon and set aside.
Add the remaining 1 tbsp oil along with the sausage rounds, onion, and red pepper. Saute them for 3-4 minutes. Add in the pasta, the broth, and the remaining spice mixture. Give everything a good stir. The broth should mostly cover everything.
Lock the lid, turn the vent to "sealing", and cook on high pressure for 4 minutes.*
While the Instant Pot comes to pressure, make the cashew cream (if using). Add the cashews and water to a high-powered blender, and blend for 1 minute or until very creamy and smooth.
When the Instant Pot timer goes off, manually release the steam. Combine the arrowroot and water to form a slurry. Turn the pot to "saute mode" and pour it in along with the shrimp. It will thicken as it bubbles.
Stir in the cashew cream or half & half and parmesan. Taste it and adjust seasonings to your liking (add salt, a little more cayenne, etc). Serve with fresh chopped green onions, if desired. Enjoy!