Make sauce by combining all its ingredients in a small bowl. Cover and refrigerate until needed.
Place all the steak ingredients, except steak, in a sided dish just large enough to hold the meat in a single layer. Add the steak and turn to coat. Cover, refrigerate and marinate 8 hours, or overnight, turning the meat occasionally.
Remove steak from the marinade and set on a plate. Discard marinade. Let steak sit at room temperature while you preheat your barbecue or indoor grill to medium-high.
Oil the bars of your grill. Grill flank steak to rare or medium-rare, about 3 to 4 minutes per side. Remove from grill and let steak rest 5 to 10 minutes. Thinly slice the steak against the grain of the meat and serve with the ginger sauce for dipping.