Coat your steak with a combination of five-spice powder and grill seasoning or salt and pepper. Heat a grill pan or griddle over high heat. Spray grill surface or wipe with a thin layer of oil. Add tuna steak to the hot cooking surface and sear tuna 2 minutes on each side. Remove tuna from heat.
Combine greens, radishes and cucumber in a bowl. In a smaller bowl, whisk wasabi, vinegar and soy sauce. Whisk in oil to combine dressing. Drizzle dressing over your salad and toss to coat evenly. Slice tuna on an angle and arrange on the salad.