Fill a large stockpot 1/2-2/3 full with water. Add tongue, onions, crushed garlic, peppercorns and salt.
Bring to a boil over medium-high heat. Once boiling, reduce to a simmer, cover and let cook for 3 hours until tender.
Using tongs, carefully remove tongue from water and allow to cool on a wire rack for 15-20 minutes (until it's cool enough to handle).
Using a sharp knife, remove the outer layer of the tongue (the part with the taste buds…I know, gross!).
Cut tongue into 1/4 inch slices. Heat avocado oil in a frying pan and sear sliced tongue on each side (about 1 minute per side) until golden brown and slightly crispy.
Chop up seared pieces of meat and serve up with desired toppings in lightly fried corn tortillas.