Cut tofu crosswise into eight 1/2-inch slices. In a 2-quart rectangular baking dish arrange slices in a single layer. Pour tamari sauce over tofu; turn slices to coat. Let stand for 15 minutes.
Meanwhile, for vinaigrette, in a blender container or food processor bowl combine vinegar, ginger, sugar, mustard, and salt. Cover and blend or process until combined. With blender or processor running, add the 1/3 cup oil in a thin steady stream. Blend or process for 15 seconds more.
Drain tofu, discarding tamari sauce. In a shallow dish combine cornmeal and sesame seeds. Carefully dip tofu slices in cornmeal mixture to lightly coat both sides. In a large nonstick skillet cook tofu in the 2 teaspoons hot oil for 5 to 6 minutes or until crisp and hot, carefully turning once. (You may need to cook tofu in two batches; avoid crowding the skillet.)
In an extra-large bowl combine spinach, watercress, mushrooms, tomatoes, and red onion. Pour the vinaigrette over spinach mixture; toss to coat. Divide among 4 dinner plates. Cut the tofu slices in half diagonally. Arrange the tofu over spinach mixture. If desired, sprinkle with additional sesame seeds. Makes 4 servings.