Preheat a grill pan over high heat for 3 to 4 minutes
Combine the mustard, onion powder, garlic powder, salt, cayenne and allspice in a small bowl. Rub the flank steak all over with the mix and let sit for 10 minutes.
Put the steak on the hot grill and cook 5 minutes per side. Remove and let rest 10 minutes before slicing.