Clean and slice the onions into long strips. Slice the peppers into long strips as well. Place peppers and onions in a large bowl, sprinkle 1½ tbsp of the fajita mix over the onions and peppers and toss.
Clean and slice the pork tenderloin into long strips and place in a bowl. Sprinkle the remaining 1½ tbsp of fajita mix over the pork and stir to make sure each piece of pork is seasoned well.
In a large skillet, heat 1 tbsp of the olive oil over medium high heat and add the pork to it. Cook the pork for 5 to 10 minutes until it's fully cooked, and it's slightly charred, but not burned.
Remove the pork from skillet, and wipe the skillet clean.
Add another tbsp of olive oil to the skillet and heat it up. Add the onion and pepper mixture and cook for about 5 minutes until the onion is caramelized and the peppers are soft. Add the pork back to the skillet, mix with peppers and onions and cook for a minute.
To make the guacamole, mash the avocados in a small bowl, then add the rest of the ingredients and stir well.
Assemble the tortillas, add some pork, onion, peppers, and top with avocado. Garnish with cilantro if preferred.