How to Make It
Fill a bowl with ice water. In a saucepan of salted boiling water, blanch the spinach until tender and bright green, 1 minute. Drain, then transfer to the ice bath. Drain the spinach and squeeze dry.
In a blender, puree the spinach, pickled ginger, miso, mirin and sansho powder with 2 tablespoons of water until smooth and spreadable.
In a small bowl, whisk the soy sauce, sake and mirin. Place the swordfish in a resealable plastic bag and pour in half of the marinade. Seal the bag and let marinate at room temperature for 15 minutes.
Meanwhile, in a small saucepan, simmer the remaining marinade over moderate heat until reduced to a glaze.
Light a grill and oil the grate. Season the swordfish with salt and pepper and grill over moderate heat, turning once, until just white throughout; baste with the glaze during the last minute or two of grilling. Cut each swordfish steak in half.
Dollop the miso sauce on plates and top with the swordfish steaks. Garnish with sesame seeds and serve.
Sansho powder is sold at most Japanese markets and online at amazon.com.