1. In a large shallow dish, combine vinegar, oil, mustard, garlic, rosemary, seasoning and pepper.
2. Add steak to the marinade mixture, turning to coat for a couple of minutes.
3. Cover the dish and place it in the refrigerator for 12 to 24 hours, turning occasionally.
4. Remove the marinated steak from the refrigerator 30 minutes before cooking so it can get to room temperature.
5. When approaching room temperature, turn on the stovetop to medium-high heat to start warming up the skillet.
6. Once the steak is at room temperature, remove it from the marinade and pat dry with a paper towel.
7. Once the steak is dry, add it to the hot skillet, cooking approximately 4 minutes per side for medium-rare doneness, so cook longer if you'd like it more well-done.
8. Once the steak is cooked, transfer it from the skillet to a cutting board and let it rest for 5 to 10 minutes.
9. Once the steak is settled, slice it thinly, against the grain, and serve!
