Heat a cast-iron or non-stick skillet over high heat. Brush the tuna steak with olive oil on both sides. Season with five-spice and chili powders, salt, and pepper. Sprinkle with sesame seeds and press gently.
When the skillet is ready, sear the tuna, about 1-2 minutes per side. Place on a cutting board and thinly slice the tuna at an angle.
Instructions for the Dressing:
Whisk together the wasabi paste, vinegar, soy sauce and olive oil. Season the dressing to taste with salt and pepper. Use some of the dressing to toss into the salad of mixed greens, radish and cucumber.
Place some salad greens on a plate. Top with the sliced tuna and drizzle with additional dressing. Sprinkle with scallions and serve immediately.