Place the basin in the 8 quart instant pot then press the sauté button. Add ghee then once hot, salt the outside of the roast then sear roast on all sides, about 5 minutes per side, until browned. Once seared, press the cancel button.
To the instant pot, add the rest of the ingredients for the first round of cooking and mix to combined. Secure the lid of the instant pot, close off the pressure valve then press manual and press the up button until the time hits 90 minutes. Once time is up, quick release to let the pressure out then remove lid and press cancel.
Remove the roast then use 2 forks to shred the roast into small pieces (you can also do this within the pot) then place the shredded beef back in the liquid. Add the rest of the ingredients for the second round of cooking, mix to combined then press the sauté button. Let mixture cook for 25-30 minutes with the lid off to reduce, until most of the liquid is cooked off.
Place each tortilla over an open flame on the stovetop to soften. Top each taco shell with a small amount of the chipotle lime mayo, then a heaping amount of the barbacoa, white onion, avocado, cilantro, and fresh lime juice on top!