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.