Using a sharp, wet knife, thinly slice tuna along grain using a long, horizontal sawing motion. The easiest way to do this is to press the fish down with one hand and cut slices off the top. Cover four serving plates with tuna. The lined plates may be reserved in refrigerator, covered with plastic wrap, up to 4 hours.
Mix vinegar, soy sauce, mirin, and ginger in a small bowl and spread about 2 tablespoons over each serving, evenly coating fish. Divide sprouts and arrange in a small fan on the side of each plate. Divide the red onion and place in a small mound at root end of sprouts. Spoon a bit of dressing over sprouts and serve immediately.
c.1997, M.S. Milliken & S. Feniger, all rights reserved