CUT the spaghetti squash through the middle rather than longways, creating two circles instead of two ovals. Use a spoon to scoop out all of the pulp and seeds.
ADD the trivet to your Instant Pot and add the water. Place the squash cut side up on the trivet. Secure the lid.
COOK at high pressure for 8 minutes and use a quick release.
MEANWHILE, heat a medium skillet over medium heat. Once hot, add 1 tablespoon oil and the pancetta. Cook for about 1 minute, until starting to crisp up. Add the mushrooms and garlic and sauté for 4 to 5 minutes, until the pancetta is crisp and the mushrooms are tender and browned. Add the sage (if using) and season with salt and pepper. Set aside.
ONCE the pressure has released, carefully remove the hot squash and dump out any water. Let cool for a few minutes until safe to handle. Run a fork around the inside of the squash, following the circle, to release the strands. Continue scraping the squash until all of the meat is free and dump into a bowl.
DRIZZLE the squash strands with 1 tablespoon oil and season with salt and pepper.
Toss. Serve topped with the mushrooms and pancetta followed by the parmesan.