Stir honey, oil, soy sauce, ginger, red pepper flakes and garlic in a medium bowl. Place flank steak and sliced onion in a large plastic zipper bag. Pour marinade into bag; seal. Refrigerate several hours or overnight, turning bag over once.
Preheat grill to MEDIUM-HIGH heat or GRILL ZONE (ORANGE). Drain meat and onions; discard marinade.
Grill meat and onions covered 7 to 8 minutes on each side. Meat will be medium rare (145°F) to medium (160°F) doneness. Let meat stand 5 minutes before slicing.