Remove beef tongue from its vacuum-sealed package and rinse lightly. Place in a large stock pot (you can use a pressure cooker or an instant pot) and add the onion, garlic, bay leaf, and thyme. Cover with the water and bring to a boil. Once it starts boiling, reduces heat and gently simmers for about 2 hours until the meat is tender.
If using a pressure cooker, cook for 40 minutes; on an instant pot, it will take about 35 minutes. Every pot or pressure cooker is different, so the cooking time will vary. The meat will be ready when it is soft enough to eat but not too cooked that it becomes mushy.
Once the meat is cooked, let it cool and remove the outer skin. Next, slice with a sharp knife; the slices should be between 1/3 and ½ in. thick.
While the meat is cooking, begin preparing the salsa by removing the husks from the tomatillos and then slightly rinsing them to remove that sticky residue that some of them have.
Place the tomatillos in a saucepan with the peppers and garlic cloves and cover with water. Turn the heat to high. Once it starts boiling, reduces the heat to low and cook for about 14-15 minutes until they are cooked.
Let the tomatillos and peppers cool after cooking and then place them in your blender. Process until you have a smooth sauce. In case you feel the salsa is too dry, add a little bit of the cooking water.
Heat the oil in a large skillet, pour the salsa in, and season with salt. Add the beef tongue slices and gently simmer for about 8-10 minutes. Right before serving, stir in the chopped cilantro.