Select saute feature on the Instant Pot, and once hot, add the ground Italian Sausage, mushrooms, and the onion. Saute until cooked.
When the sausage is cooked, at the salt, oregano, Brussels sprouts. Stir for 1 minute until the shaved sprouts cook down.
Remove the stuffing mixture from the Instant Pot liner and place it into a large bowl.
Add the Parmesan to the mixture and stir. Cover with foil and set aside.
Pour 1 cup of water into your Instant Pot liner.
Place your trivet (it came with your Instant Pot) into the water.
Stack your acorn squash halves into the pot.
Secure the lid, set to sealing, and set the function to STEAM for 6 minutes.
When the pressure cycle is complete, quick release the pressure.
Carefully remove the squash from the pot using tongs.
Stuff the squash with the sausage mixture.
OPTIONAL: broil the stuffed squashes if you prefer a crispy topping.