Heat a large cast iron skillet over high heat and add just enough avocado oil to coat the surface. Once the skillet is very hot, place the lamb shanks on the surface and sear for 5 to 8 minutes.
While the lamb shanks are searing, prepare the sauce. Plug in your Instant Pot and press the Sauté button and allow it to heat up for a few minutes. Add 1 tablespoon of avocado oil, then add the onion.