Place the tuna on a cutting board and slice it into small cubes.
In a small bowl, combine all the Ingredients for the sesame-soy sauce (soy sauce, rice vinegar, sesame oil, honey, and wasabi paste).
Add the cubed tuna and cover the bowl. Keep chilled in the fridge for 15 minutes.
In a large bowl, combine the marinated tuna, cucumber, mango, and avocado. Sprinkle with basil leaves and toasted sesame seeds.