In a large bowl, whisk together the sauce ingredients until combined. Set aside.
Lightly rub the short ribs with salt. Hit Sauté on the Instant Pot and Adjust so it’s on the More or High setting. After 5 Minutes of the pot heating, working in batches, flash-sear the short ribs for 30 seconds on each side. Set aside on a plate to rest.
Pour about 1⁄4 cup of the sauce into the pot and deglaze (scrape the bottom of the pot) to get up any browned bits. Then, nestle in the short ribs, top with the onions, and pour the remaining sauce on top. Secure the lid, move the valve to the sealing position, hit Cancel, and then hit Manual or Pressure Cook on High Pressure for 45 Minutes. When done, allow a 15-Minute natural release followed by a quick release.
Meanwhile, make a slurry by mixing together the cornstarch and water.
Use tongs to carefully transfer the short ribs to a serving dish (they’ll be so tender the meat will likely slide right off the bone). Hit the Cancel button, then hit Sauté and Adjust so it’s on the More or High setting.
As the sauce is coming to a bubble, stir in the slurry and onion dip mix (if using). The sauce will thicken almost immediately. Hit Cancel to turn the pot off.
Ladle the sauce over the short ribs and enjoy with Truffle Hash (page 240) or Aligot (page 228).