Preheat grill. Brush steaks with oil and season with salt and pepper to taste. Grill for 1 minute on each side (just to obtain grill marks) Remove and slice each steak into 8 slices.
Place 1 cup of mizuna on each plate. Divide avocado and tomatoes among plates. Drizzle with the wasabi vinaigrette.
Whisk together all ingredients until combined. Season with salt and pepper to taste