Immerse plank in water, placing a weight on top of plank to keep it submerged. Soak at least 4 hours or overnight. Meanwhile, mix dressing, onions, parsley, cilantro, garlic and red pepper.
Remove 1/2 cup of the dressing mixture; set aside for later use.
Pour remaining dressing mixture into large resealable plastic bag; add steak. Seal bag; turn over several times to evenly coat steak with the dressing mixture. Refrigerate at least 2 hours to marinate.
Preheat grill to high heat.
Remove steak from marinade; discard bag and marinade. Grill steak 3 min. on each side; remove from grill.
Remove plank from water; brush top with oil. Top with steak.
Place on grate of grill; cover with lid. Reduce grill to medium heat.
Grill 15 to 20 min. or until medium doneness (160F).
Remove steak from grill; discard plank. Cover steak loosely with foil; let stand 5 min.
Cut steak into thin slices. Spoon reserved dressing mixture over steak.