Wrap the flank steak in plastic wrap, and place in the freezer for 3 hours.
Unwrap and slice across the grain into ¼-inch thick slices.
In a medium bowl, combine all the "Marinade" ingredients. Add them to a gallon size Ziploc bag, and add the steak slices.
Marinate for at least 2-3 hours or overnight, turning the bag occasionally.
Place a large cast-iron grill pan over medium-high heat and once heated through add 1 tablespoon of vegetable oil.
Once the oil is hot, working in batches, add the steak slices to the pan in a single layer and cook until charred, flipping once, it takes about 2-3 minutes per side to cook.
Add the remaining 1 tablespoon of vegetable oil and cook the rest of the steak.
Serve immediately over white rice. If desired, garnished it with chopped green onions and toasted sesame seeds.
