In a small bowl, combine the first 6 ingredients (oil through salt). Place the flank steaks and rosemary in a large, re-sealable plastic bag and pour the marinade over it making sure the entire steak is covered. Marinate for at least 4 hours or overnight, turning the bag several times.
When ready to cook, wipe the steak dry with a paper towel. Season both sides with salt and pepper.
In a large skillet, heat the oil over high heat. Add the steak and let it cook, undisturbed for 4 minutes.
Using tongs, turn the steak and let cook another 4 minutes. For medium rare, the meat thermometer should register 135° or be slightly firm to the touch.
Let rest for 5 minutes before cutting into thin slices, being sure to cut against the grain.