Place whole tongue and water into Instant Pot. (See Recipe Notes below if you don't have an IP.) Seal lid and close valve. Select Stew setting. IP will cook for 35 minutes. Allow pressure to release naturally for 30 minutes, then put dish towel over valve and release pressure.
Using tongs, remove tongue to cutting board to cool slightly. When cool enough to handle, peel tongue, making a cut through the skin to begin. (See photo.)
Slice tongue starting at the tip, in 1/2" slices, at a slight angle to get larger pieces at the tip.
Heat large cast iron skillet or other pan over medium-high heat. Add 1 Tablespoon fat to pan, spreading it around. Place meat slices closely together. Sprinkle with 1 tsp. sea salt, freshly ground pepper, to taste, and optional spices. Cook for 5 minutes, then reduce heat to medium for an additional 3 minutes. Check the surface that's frying. When it's crispy flip each piece.
Lightly salt second side of meat. Fry on second side until crispy. Remove first batch and start second batch of meat frying: repeat frying steps with any meat that remains, using lard, sea salt and spices as you did with the first batch.
Remove meat to a cutting board and cut into thin strips, as desired. Serve in Mexican food settings, such as tacos, big salads, over eggs with green chilies, inside soft tortillas etc. with accompaniments: fresh cilantro, sweet onions, salsa, avocado, sour cream, cheese, fresh radishes etc.
If you don't have an Instant Pot, simply place tongue(s) in large pot. Cover with water. Add 2 teaspoons sea salt. Bring water to a boil, then reduce heat to a simmer. Simmer 1 hour for every pound of meat. This will likely be a minimum of 3 hours. A knife needs to slide to the center of the meat easily, to indicate the meat is tender. If in doubt, cook 1 additional hour. A crock pot can also be used for this step: simply slow-cook the tongue in the salted water all day, as you would a roast.