Mix soy sauce, wasabi, sesame oil and green onion together in a small bowl. Add diced tuna and stir to coat. Cover with plastic wrap and place in refrigerator to marinate for 2 hours. Drain the tuna; discard marinade.
To construct the tower, mound and flatten the cucumber in the center of a serving platter. Top the cucumber with mango and then the avocado. Place the marinated tuna on top. Sprinkle with toasted sesame seeds.
To serve, place chips around the poke tower for scooping. Or serve with hot sticky (sushi) rice, or enjoy as is as a salad.
