Whisk together the molasses, vinegar, mustard, soy sauce, olive oil, garlic, and rosemary in a 2 cups measuring cup. Pour the marinade into a 1-gallon size ziptop bag. Add the flank steak and turn to coat with the marinade. Place in the refrigerator for 4 hours (or at least 1 hour if time is limited). Remove the steak from the fridge at least 30 minutes before grilling to remove the chill.
Heat a cast iron grill pan or outdoor gas grill over medium high heat. Remove steak from the marinade letting excess drip off. Season the steak well with salt and pepper. Grill steak on each side for 5 to 6 minutes, a total of 12 minutes. Let rest for 10 minutes before slicing across the grain.