Place beef tongue in a large stock pot. Add onions, garlic, salt, peppercorn, bay leaves and jalapeno pepper. Cover with water. Bring to boil. Then, reduce to simmer, cover and cook for 4 hours until tongue is tender
Remove beef tongue from water and allow to cool
Peel off the tough outer skin using your fingers or a small sharp knife.
Remove patch of meat where tongue would connect to mouth.
Slice tongue (approximately 1/4 inch slices)
Heat vegetable oil in large skillet on medium-high heat. Saute the tongue slices until they are lightly browned on both sides.
Remove from skillet, and dice beef into small cubes. Alternatively, you can shred the beef using two forks.
Make the tacos by placing a large spoonful of beef in each corn tortilla. Add other desired toppings like avocado, onion and cilantro.