1. For the steak, add miso, mustard, ginger, red pepper flakes, salt, vinegar, garlic, and vegetable oil to a small bowl. Whisk to combine into a paste. Spread both sides of the steak with the paste and place in a plastic sealable bag. Chill for at least 30 minutes, up to overnight in the fridge.
2. When ready to cook, preheat oven to 450˚F. Brush a baking sheet with vegetable oil. Remove steak from marinade and place on prepared sheet pan.
3. Roast for 15 minutes, until medium rare, turning halfway through. Let rest for 10 minutes on the baking sheet before removing to a cutting board to thinly slice against the grain.
4. While the steak is roasting, whisk together all ingredients for the dressing. Set aside.
5. To make the salad, mix together greens, cherry tomatoes, avocado and green onion in a large serving bowl. Top with sliced steak and pour over dressing.