I've seen similar recipes on several websites, and they're all quite similar. It makes a lovely presentation when served on a bed of fresh baby spinach or butter lettuce, and it's really delicious and quite easy to prepare. I hope you give it a try.
Whisk together the miso, mustard, ginger, salt, garlic and 2 tablespoons oil until a paste forms. Rub paste on both sides of the flank steak. Wrap well in plastic and let marinate in refrigerator for at least 30 minutes or up to 8 hours.
Arrange rack in upper third of oven, and preheat oven to 450°F.
Brush a baking sheet with oil. Remove steak from marinade, shaking off excess marinade. Place steak on prepared baking sheet.
Roast in preheated oven for about 13 minutes (for medium), turning once. Turn on broiler, and cook steak under broiler, watching closely to prevent burning, for about 1 minute per side or until the surface turns a deep brown. Remove from oven and let rest 10 minutes. Season with sea salt and black pepper, as desired.
Thinly slice against the grain, and serve immediately.