Pat pork tenderloin dry with a paper towel.
Brush the Dijon mustard evenly all over the surface of the pork.
In a small bowl, mix together the minced garlic, brown sugar substitute, and parsley. Rub this mixture all over the pork.
Cover and refrigerate for at least 30 minutes, up to overnight, to let the flavors develop.
When you're ready to cook, set the Instant Pot to sauté. Add the oil. Once hot, sear the pork tenderloin on all sides, about 1–2 minutes per side, until lightly browned.
Cancel sauté. Add the water to the pot, scraping up any browned bits from the bottom with a wooden spoon.
Lock the lid in place, set the valve to sealing, and pressure cook on low for 5 minutes.
Once the cooking cycle ends, allow the pressure to naturally release for 10 minutes before turning the valve to venting to release any remaining pressure. If your tenderloin is closer to 2 1/2 pounds, allow natural release for 12 minutes before venting.
Carefully open the lid. Transfer the pork to a platter and cover loosely with foil. Let rest for 10 minutes.
Return the pot to sauté mode. Once the cooking juices begin to simmer, stir in the apple cider vinegar. Sprinkle the xanthan gum over the liquid and whisk constantly until the sauce thickens, 1–2 minutes.
Slice tenderloin into equal-sized medallions. Drizzle with the sauce and serve.