Although you may need to make a trip to the Asian market to pick up miso and mirin, look for wasabi powder in your supermarket's spice aisle; it's much milder than the kind found in Japanese markets.
Combine first 5 ingredients in a small bowl; stir well with a whisk. Combine miso mixture and steak in a large zip-top plastic bag. Seal and marinate in refrigerator 2 hours.
Prepare grill or broiler.
Remove steak from bag, reserving marinade. Place steak on grill rack or broiler pan coated with cooking spray. Grill or broil 6 minutes on each side or until desired degree of doneness, basting occasionally with reserved marinade.
This is one of my go to recipes and even when I try other flank steak marinates, we always return to this one. It is a great general marinade - doesn't necessarily taste Japanese so I use it all the time with a variety of side dishes.