In a small bowl, mix the taco seasoning with the oil until a loose paste forms. Pour that paste over the flank steak and rub into the meat on both sides.Let that marinate for at least 30 min and up to overnight. The longer it sits, the more flavorful and tender it will be.
When you are ready to make the steak, preheat the broiler as high as it will go. Cover a sheet pan with aluminum foil and place the flank steak on a rack and place it on the sheet pan. Tip-If you pull the meat out of the fridge at least 30 minutes prior to cooking it and bring it to room temp it will cook more evenly.
Season the steak with salt and pepper to taste, be slightly easy with the salt as the taco seasoning can be slightly salty.
Place under the broiler and cook it to the desired temperature. Let it get nice and charred a bit on the outside.Once it has reached the temperature you are looking for; Tip- 8 min on each side for medium rare. Pull out of the broiler and let the meat rest for at least 10-15 minutes to let the juices redistribute.
Slice the steak against the grain and serve it warm.
Optional accompaniments-Corn and flour tortillas, shredded cheese, shredded lettuce, guacamole, sour cream and fresh cilantro