In a zipper-lock bag, add the oil, lime juice, cilantro, cumin, salt, and pepper. Mix everything together and add the steak to the bag. Marinate for about 15 minutes at room temperature.
Warm a pan over medium-high heat. Add the steak to the pan and cook for four to five minutes on each side, or until desired doneness. Remove the steak from the heat and allow it to rest for about five minutes before slicing into it.
Cut the steak into small cubes. Divide the steak and guacamole between tortillas. Top with onion and cilantro. Enjoy!