Stir together the diced onion, minced garlic, and oil in a microwave safe bowl. Cover with a paper towel (to prevent splatters) and microwave until softened, about 5 minutes. (Alternatively, you can saute them in the Instant Pot, adding the garlic during the last minute of cooking time.) 

Place the pork loin, soy sauce, maple syrup, ginger, cayenne, and cooked onion mixture in a gallon-size freezer bag. Seal tightly and toss to combine. 

Marinate the pork in the refrigerator for at least 2 hours and up to 24 hours.

Transfer the pork and marinade to the Instant Pot. Lock and seal the lid. Cook at high pressure for 25 minutes, then allow the pressure to release naturally, about 10 minutes. (Check to make sure the pork is at least 145°F internally. If not, place it back in the Instant Pot on the Warm setting, close the lid, and check the temp every 5 minutes.) Transfer the pork to a cutting board, tent with foil, and let rest for about 10 minutes.

Meanwhile, set the Instant Pot to “Sauté.” In a small bowl, stir together the cornstarch and 2 tablespoons of water until smooth. Stir the cornstarch mixture into the sauce remaining in the pot and bring to a boil. Cook, whisking, until thickened, about 3 minutes. Press “Cancel.” Taste and season with salt and black pepper, if desired.

Cut the pork into 1/2-inch thick slices. Fan them out on a serving platter and drizzle the sauce over the meat. Serve.