Place the tongue, onions, garlic, bone broth, and water into your instant pot insert (or slow cooker or a medium saucepan).
Seal your instant pot, and turn on to manual, high pressure, for 45 minutes. Allow for a natural pressure release. (For slow cooker, add enough water to completely cover the tongue and cook for 10 hours on low.) (For stovetop, add enough water to completely cover the tongue, bring to a boil, reduce heat and simmer for 3 hours.)The tongue should be very tender at this point, and a sharp knife should slip into the meat easily. If it doesn't, cook it a little longer.
Remove the tongue to a cutting board and allow to cool slightly. Pour the cooking liquid through a strainer into a bowl. Set aside the onion mixture from the strainer and 1/2 cup of the cooking liquid. If desired, you may save the remaining cooking liquid for up to a week in the refrigerator or up to a month in the freezer and use in any recipe calling for beef broth.
Peel the skin off the tongue and discard. The part of the tongue that attaches to the bottom of the mouth will be tough, so cut it off and discard it. You can easily feel where the tough part is by squeezing it. Dice the meat as finely as you would like.
Return the diced meat, the strained onion mixture, and 1/2 cup of the cooking liquid to the instant pot insert. Add the taco seasoning and diced green chiles (if using). Press the saute button on the instant pot. Cook the meat mixture, stirring ocasionally, until most of the liquid has evaporated. Serve with your favorite keto tortillas and toppings!(If you used a slow cooker to cook the tongue, complete this step in a skillet on the stovetop over medium heat. If you used a saucepan to cook the tongue, complete this step in that same saucepan on the stovetop over medium heat.)
Notes
The nutrition facts are only for the meat mixture. They include neither the tortillas nor the optional toppings.