Place the nopales on a grill or grill pan over medium heat. Let them cook for 6-8 minutes on each side or until they turn olive green. They should also have some grill marks and have begun to release a bit of their slime out the sides. The exact cooking time will depend on the size and thickness of the nopales, but they should have softened.
Remove them from the heat and set them on a cutting board. Use a sharp knife to cut them into 1/2" thick strips. Set them aside.
In a medium sized bowl, mix the onion, tomato, serrano pepper, and cilantro until well combined. Add the sliced nopales and mix well. Season with salt to taste.
Place 3-4 tortillas in a large griddle or pan over low heat. You can also use a comal if you have one. Don't overlap them too much so they heat evenly. Let them heat for 2-3 minutes on each side or until soft and bendy. Remove them from the heat and set them aside. Cover them with a clean kitchen towel to keep them warm.
Assemble the tacos with warm corn tortillas and the nopales asados mixture. Add cotija cheese and salsa to taste.
*Serrano Pepper: If you don't like heat, swap out the serrano for a jalapeno. Make sure to remove the seeds and membrane from peppers if you don't like a lot of heat.