Whisk together the molasses, mustard, olive oil, vinegar, soy sauce, rosemary and garlic in a 2-cup 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 the steak from the marinade, letting the excess drip off.
Sprinkle the steak well with salt and pepper.
Grill the steak on each side for 5 to 6 minutes, a total of 12 minutes.
Let rest for 10 minutes before slicing across the grain.