To make it tender put the cow's tongue, peeled onion, cut into halves, carrot slices, bay leaf, salt, allspice and coriander into a pot. Pour in the beef broth. If it doesn't cover the tongue, add more broth or water. After it begins boiling put in the cow's tongue and boil on medium heat covered with a lid for 3 hours.
When the tongue is cooked, take it out of the broth and cool. Peel or cut the outer tougher layer off and discard. Then cut the insides into thin strips. Melt the butter in a frying pan and fry pieces for 5-7 minutes until browned to your liking.
Tortillas can also be fried in a pan to make the edges crispy. Put the fried tongue on the tortilla, salsa verde on top. Add chopped tomatoes and cilantro to tortillas. Drizzle with lime juice, if desired, before serving.