Prepare the marinade by combining the balsamic vinegar, rosemary, mustard, garlic, salt, and pepper in a blender or food processer. Slowly drizzle in 4 tbls of olive oil.

Place the flank steak into a Ziploc bag and pour in enough marinade to fully coat the steak. Reserve any extra marinade on the side to use as a salad or vegetable dressing while serving. Place the Ziploc bag in the refrigerator and marinate 4-24 hours.

Remove the steak from the marinade and put onto plate, letting excess marinade run off. Let sit at room temp for 10 minutes. Just before cooking, season steak with salt and pepper, to taste.

Heat a medium sized skillet over medium/high heat. Add 1 tsp olive oil and cook steak 3-4 minutes on one side, flip, and cook an additional 3-4 minutes on the other side until desired doneness. Transfer to cutting board and cover with foil to let rest for 5 minutes.

Slice steak against the grain and serve.