1Whisk together miso, vinegar, oil and chile flakes in a small bowl. Reserve 1/2 cup. Transfer remaining mixture to a large resealable plastic bag and add 1 tsp. each garlic and ginger.
Add sugar and remaining 1/2 tsp. each of garlic and ginger to reserved miso dressing; chill until ready to use.
Add steak to bag and seal. Carefully turn bag over several times and massage steak to coat well. Chill 8-24 hours.
2Preheat grill to high (500-550 degrees). Remove steak from marinade, letting excess drip off. Discard marinade. Grill steak, covered, about 3 minutes per side for medium-rare. Transfer to cutting board and let rest 10 minutes.
3Whisk 2 tbsp. of water into reserved miso dressing. In a large bowl, toss lettuce and radishes with 3 tbsp. dressing.
4Thinly slice steak and arrange on a platter with salad. Drizzles with any juices from the cutting board, along with the remaining miso dressing.