I adapted this recipe from Serious Eats, one my new favorite food Web sites. I decreased the soy called for and increased the mirin, and also doubled the marinade because my steak was twice as big as the one called for. Seriously good, Ernie was already planning when to eat the leftovers.
2 tablespoon miso paste, (I used shiro-miso)
3 tablespoon mirin
1 teaspoon soy sauce
2 teaspoon sesame oil
1 teaspoon chili pepper
1/2 inch piece of ginger, grated
Flank steak (mine was about 1.8 pounds)
Add the miso paste, mirin, soy sauce, sesame oil, chili pepper, and grated ginger to a bowl. Whisk until combined.
Add the meat to the bowl and toss until well coated. Let marinate for about 30 minutes, flipping the meat about halfway through.
At this point, I threw this on the grill (set on high) for about 5 minutes on each side.
Let the steak rest for a few minutes, and then slice against the grain into 1/4 inch pieces.