�2 tablespoons wasabi powder (horseradish powder)*
�1 1/2 tablespoons water
�1/2 cup mayonnaise
�1 pound fresh tuna steaks, cut into 3/4-inch cubes
�2 1/2 tablespoons soy sauce
�28 large slices pickled ginger*
�28 8-inch wooden skewers
�1 bunch watercress
�1 teaspoon freshly ground black pepper
�1 tablespoon vegetable oil
�Mix 2 tablespoons wasabi powder and 1 1/2 tablespoons water in small bowl to blend. Whisk in 1/2 cup mayonnaise. Cover wasabi mayonnaise and refrigerate at least 30 minutes. DO AHEAD Can be made 1 day ahead. Keep refrigerated.
�Combine tuna and soy sauce in medium bowl; toss to coat. Marinate tuna 30 minutes at room temperature, stirring occasionally.
�Meanwhile, thread 1 ginger slice onto each skewer 2 inches from tip.
�Line platter with watercress. Place bowl of wasabi mayonnaise on platter. Drain tuna; pat dry. Return to medium bowl. Sprinkle with pepper; toss to coat. Heat oil in large skillet over medium-high heat. Add tuna and sear until browned on all sides but still pink inside, about 2 minutes total. Thread 1 tuna cube onto each prepared skewer next to ginger. Arrange skewers on platter and serve.