Step 1: Flank Steak Marinade
1/2 cup soy
1/2 cup white sugar
1/2 cup water
1/4 cup sesame oil
1/4 cup sake
2 tablespoons chopped garlic (or more if you like it more garlicky)
1 tablespoon sesame seeds
1 teaspoon black pepper
1 bunch of scallions, washed and rinsed well. (This will be simply added in between the beef later for the final layer of flavor. I used to chop scallions into the marinade, but would end up with too many charred bits of scallion which just wasn't that appetizing. If you have no scallion, feel free to throw in some white onion wedges in with the beef after marinating.)
Set aside scallions and meat. Mix everything else together in a large bowl (marinade ingredients.)
Quickly rinse the meat (this is a Korean thing as my mother always insist that it all needs to be rinsed JUST IN CASE...of what I'm unclear. I do believe in rinsing kalbi for sure because shards of bone often end up on the meat and washing that away is a good thing.)
Lay the flank down on a cutting board. It is a pretty wide flap of meat, and it will be hard to maneuver on the bbq in this form. Cut in half ALONG the grain - down the center, and you will have two halves.
Dunk each piece of steak into the marinade, making sure to submerge it and coat all sides of it. (I wear a disposable glove because sesame oil and soy sauce makes your hand smell forever.) Remove from marinade and place into storage container.
Layer in green onions as you place in the flank. Continue until all flank is marinated. Pour remaining marinade on top.
Marinate at least 8 hours or overnight if possible. I personally have found that a 24 hour soak yields the best flavor. If you are planning to freeze, you can allow it to soak and then portion off what you want into ziploc bags OR immediately put into bags and freeze. Try and put a bit of marinade into each bag when freezing. I generally freeze after I've cooked a batch only because people eat more than I expect them to - and I always end up freezing less than I would have.
You can grill it in the bbq (our family favorite) or broil it in the oven for about 12-15 minutes, depending on your preference for rare, medium or well.