The night before you grill, combine all the marinade ingredients in a large, deep sided skillet.
Bring the marinade to a boil and simmer for about 5-10 minutes to boil off the alcohol.
Meanwhile, slice the flank steak into thin strips, about 1/4 inch thick.
Let the marinade cool to room temperature. Pour half of the mixture into a container with your sliced beef to marinate overnight in the fridge.
The remaining half of the marinade is for teriyaki sauce. Whenever you are ready to make the sauce, bring the liquid to a simmer again in a skillet over stovetop.
Keep simmering for about 10 minutes or until the liquid is reduced to a shiny glaze, almost the consistency of maple syrup. Remember that this glaze will thicken slightly as it cools.
If making ahead of time, store the sauce in an airtight container in the fridge.
After the beef has marinated overnight, it's ready to grill. Preheat your grill and remove the beef from the marinade.
Grill on medium high heat for 3-5 minutes per side. Remember cook times will vary depending on how thinly you were able to slice your steak.
Plate your beef with rice, spooning the teriyaki sauce on top, and serve! Leftovers can be kept in an airtight container in the refrigerator and reheated in the microwave.