Pat the flank steak off so that it is nice and dry.
Into a small bowl, mix the rosemary, thyme, garlic, red pepper flakes, salt, and pepper.
Using 1 Tbsp of Olive oil and a brush, spread all over the dried steak.
Sprinkle the herb mixture all over steak on both sides, pressing in to make sure it sticks.
Let sit while the steak comes to room temperature.
Heat your cast iron skillet up to high. Once hot, add the olive oil and the butter, then add the flank steak to the hot skillet.
For medium, cook for about 5 minutes on each side.
Remove the steak from the skillet and put on a cutting board or plate. Cover with foil and let rest for 5-7 minutes. I generally prefer to use a plate so that I can catch all of the delicious juice as it rests and pour over after cutting.
Cut the steak against the long muscular grains.