This recipe comes to us courtesy of Jill Norman's fabulous spice book, "Herbs and Spices: The Cook's Reference." It's the best book on spices and herbs we've seen yet, and it has great recipes as well.
Peel alternate strips of the cucumber, cut it in half lengthwise, and remove the seeds. Slice very thinly and spread in a colander. Sprinkle the layers of cucumber with salt and let drain for 1 hour.
Remove the skin from the tuna and dice the flesh with a very sharp knife. Put the flesh into a bowl. Combine the lemon juice and wasabi with a little salt and pour over the fish. Lift the fish with your fingers to mix it well with the dressing. Cover, refrigerate, and let marinate for 1 hour.
Rinse the cucumber and dry on a cloth. Make a vinaigrette with the vinegar, olive oil, and pepper, and stir in the cucumber and chives. Spoon a flattish circle of cucumber onto a serving plate. Drain the tuna and mound it on top. Garnish with the mitsuba or parsley.