Unfold the flank steak and place in a self sealing bag or glass dish. Mix the oils, vinegar, soy sauce, garlic, sugar, salt, pepper and sesame seeds together. Pour over the steak. Make sure both sides are completely marinated and refrigerate for at least 2 hours.

Pre heat grill. Remove the steak from the refrigerator and let sit at room temperature for 15 minutes. Shake off excess liquid and grill on both sides until optimal doneness is reached.

Let steak stand 5 minutes before slicing. To help prevent chewiness in the meat be sure to cut thin slices against the grain.