Season the flank steak all over with salt and 2 teaspoons of the oil and allow to rest in the fridge uncovered overnight, or for at least a few hours.
Remove the flank steak from the fridge and allow to come to room temperature before cooking.
Heat a 12 inch cast iron skillet until smoking hot, adding the remaining teaspoon of oil to the pan and rubbing around with a towel so the pan is evenly greased.
Press both sides of the steak in the blackening spice, then put in the pan and cook on high for 1 minute per side, or until blackened.
Remove the steak to a paper towel to weep liquid and keep in a warm place for 5 minutes.