Place the beef tongue in a slow cooker and cover with water. Add the onion slices, garlic, and bay leaf. Season with salt. Cover and cook on Low overnight or 8 hours. Remove the tongue and shred the meat into strands.
Heat the oil in a skillet over medium heat. Cook the tomatoes and peppers in the hot oil until softened on all sides. Remove the tomatoes and peppers in a blender, keeping the oil on the heat; season with salt. Blend briefly until still slightly chunky. Cook the diced onion in the skillet until translucent; stir in the tomato mixture. Cook another 5 to 6 minutes. Build the tacos by placing shredded tongue meat into a tortilla and spooning salsa over the meat.
