To prepare the marinade, add olive oil, balsamic vinegar, shallot, garlic, rosemary, 1 teaspoon of kosher salt, and ½ teaspoon pepper to a small bowl. Mix well.
Pour marinade into a large Ziploc bag. Add flank steak to bag, then seal, and massage marinade into steak. Refrigerate for a minimum of 4 hours to overnight.
Preheat grill to 450 F. As it preheats, remove the steak from the marinade. Season steak on both sides with remaining salt and pepper.
Grill steak over direct heat until well seared on one side, approximately 7 minutes.
Using tongs, flip the steak and continue grilling until internal temperature reads 130 F, approximately 3 to 5 minutes more depending on the thickness of the steak.
Remove from the grill and place on a cutting board. Cover with aluminum foil and let rest for 10 minutes.
Slice steak thinly across the grain and serve immediately.