Adapted from bon appétit
2 tablespoons wasabi powder or wasabi sauce
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
1 tablespoon Dijon mustard
28 large slices pickled ginger*
28 (8-inch) wooden skewers
1 bunch arrugula
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.
Combine tuna, soy sauce, and Dijon mustard in a medium bowl; toss to coat. Marinate tuna 30 minutes at room temperature, stirring occasionally.
Thread 1 ginger slice onto each skewer 2 inches from tip.
Line platter with arrugula. 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.