Combine the tuna strips with the soy sauce, wasabi and sake. Cover and chill until required.
Arrange the salad leaves, tomatoes and cucumber on a large serving plate. Cover and chill until required. Mix the dressing ingredients in a bowl and set aside.
When ready to serve, heat a non-stick pan over a high heat and fry the tuna strips for about 10 seconds on each side, or until seared. Arrange the tuna on top of the salad, drizzle with the dressing and serve immediately.