Place beef tongue, half the onion, garlic cloves and bay leaves in a slow cooker; season with salt and freshly ground pepper. Add in just enough water to cover all the ingredients.
Set the slow cooker to LOW and cook for 8 hours.
Take the beef tongue out of the slow cooker. Transfer it to a large chopping board and let it cool slightly. Discard the rest of the slow cooker contents. Peel the outer layer of skin from beef tongue and cut off the rough end.
Using two forks, shred the meat and remove any tough connective tissue/membrane as you go.
Heat lard/butter in a large skillet and saute the chopped Serrano chili until fragrant, 1 to 2 minutes. Over medium heat, add the shredded meat, the chipotle and garlic powders and salt and pepper to taste and stir to mix well, for two to three minutes tops. Do not overcook and don't let the meat get dry.
Warm up the soft corn tortillas, make the tacos, garnish with chopped onion and cilantro and top with the salsa. Serve warm and enjoy!
