How to Make It
Whisk 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 garlic and ginger to reserved miso dressing; chill until ready to use. Add steak to bag and seal, squishing out air. Carefully turn bag over several times and massage steak to coat well. Chill 8 to 24 hours.
Preheat grill to high (500° to 550°). Remove steak from marinade, letting excess drip off, and discard marinade. Grill steak, covered, about 3 minutes per side for medium-rare. Transfer to a cutting board and let rest 10 minutes.
Meanwhile, whisk 2 tbsp. water into reserved miso dressing. In a large bowl, toss lettuce and radishes with 3 tbsp. reserved dressing.
Thinly slice steak and arrange on a platter with salad. Drizzle with any juices from the cutting board, along with remaining miso dressing.