Preheat the gas grill to 300 °C and lightly brush the flank steak with the sunflower oil.
Grill the steak hot for 90 seconds on each side. If you want a grill pattern, turn the flank steak 90 degrees after 50 seconds.
Turn and grill again for 90 seconds.
Grill at a temperature of around 140 °C to the desired core temperature. 56 °C for medium, 60 °C for done (cooked all the way through).
Use salt and pepper to taste.