Put dry Hidden Valley Ranch Mix in a small bowl with the cumin.
Mix in the lime juice and oil. Mix well with a wire whisk.
Pour a little marinade in the bottom of a shallow dish. Place flank steak in the dish and pour the rest of the marinade over the top. Cover with plastic wrap and marinate overnight in the refrigerator.
Cook the steak on the grill, in a frying pan or under the broiler. Cook for eight minutes on each side, four minutes at a time, turn over and then do that again.
Once cooked, rest for 3-5 minutes with tin foil on top. Slice the steak across the grain, and serve immediately.