Brush the tuna with sesame oil and season well with salt and pepper. Cook on a hot barbecue for 10 seconds on each side, then rinse under cold water to stop the fish cooking further. Pat dry and set aside.
Stir the dressing ingredients together in a bowl and season to taste.
Blanch the asparagus in lightly salted boiling water for 2 minutes. Drain well, refresh under cold water and pat dry.
Arrange the salad leaves and asparagus on a large platter. Slice the tuna thinly and arrange with the salad. Drizzle with the dressing and serve.