The longer the flank steak marinates, the more flavorful it will be. Use the ziploc bag to keep the meat and vegetables fully coated in the marinade. Be sure to cut against the grain when slicing the meat. For the dipping sauce, never use the marinade from the steak; instead set aside a separate, unused portion of the sauce from the outset.
With a fork, prick the flank steak all over. Season both sides of the steak with the salt and pepper.
In a small bowl, whisk the pomegranate molasses, olive oil, mustard, brown sugar, and garlic powder. Set aside about 1/3 of the sauce to use when serving the meat after grilling.
Pour the rest of the marinade in a gallon ziploc bag. Add the flank steak, pepper slices, and onion slices. Pushing out as much air as possible, close the bag and refrigerate the meat for at least 12 hours and up to 24 hours.
To grill the meat, coat the grill grates with the neutral oil using tongs and a bit of paper towel. Heat the grill to medium high.
Once the grill is hot, lay the flank steak, pepper and onion on the oil-coated grates. Grill for 6- to 8 minutes, then flip the steak and vegetables. Grill for another 6 minutes, or to 150 degrees at the center of the steak.
Brush the finished steak with some of the reserved sauce while it rests for 15 minutes. Slice the steak on an angle against the grain, and serve with more of the reserved sauce over top or on the side.