In a food processor, combine the cilantro, olive oil, garlic, lime juice, and salt. Grind until almost smooth.
Place the flank steak in a plastic bag and rub the cilantro mixture all over both sides. Refrigerate the meat for at least an hour and up to 24 hours. The longer it sits, the stronger the flavor.
When ready to cook, place the flank steak with the cilantro paste still on the meat onto a broiler pan. Place the broiler pan about 6 inches from the heat and broil under high heat. For medium-rare about 5 minutes, then flip and broil another 5 minutes. Remove from the broiler and let the meat rest 10 minutes before slicing.
Serve with tortillas, cheese, sour cream, lettuce, tomatoes, and onions for an easy taco buffet.