1. Pierce each potato a few times with a knife, place in a microwave-safe dish and loosely cover with plastic wrap. Microwave on high 5 to 6 minutes or until potatoes are fork-tender. Microwave green beans according to package directions.
2. Heat a grill pan over medium-high heat. Coat with cooking spray; season steak with salt and pepper and cook 2 to 3 minutes a side or until desired degree of doneness. Transfer to a cutting board to rest.
3. While steak is cooking, combine miso, vinegar, ginger, sugar and sesame oil in a small bowl. Whisk in vegetable oil.
4. Cut potatoes in half, then cut steak into 1/4-inch slices. Divide potatoes, steak and beans among four plates; drizzle with dressing. Sprinkle scallions over steak and sesame seeds over potatoes, if desired.