Place the soy sauce, wasabi and sake or white wine in a non-metallic bowl and mix well. Add the tuna, turn in the marinade to coat and leave to stand for 10 minutes.
Divide the salad leaves, tomatoes and cucumber between 2 serving plates.
Place all the dressing ingredients in a screw-top jar and shake to combine.
Heat a nonstick frying pan over a high heat until very hot, add the tuna slices and fry for about 10 seconds on each side or until seared. Arrange the tuna on top of the salads, drizzle with the dressing and serve immediately.