a shallow glass dish, combine 1 tablespoon of
the soy sauce and 1 teaspoon of the oil. Add
the tuna and coat well. Refrigerate for about
15 minutes, turning the fish twice.
a grill or preheat the broiler. In a small bowl,
mix the wasabi powder with 4 teaspoons of hot
water. Cover and set aside for 5 minutes.
another bowl, stir 2 tablespoons of warm water
into the tahini. Stir in the wasabi paste, vinegar,
mustard, sugar, garlic and the remaining 2 teaspoons
soy sauce and 4 teaspoons oil. Season with salt
the tuna with salt and pepper; grill or broil
until crusty on the outside and rare within,
about 4 minutes per side. Transfer the fish
to a plate to rest for a few minutes, then slice
it against the grain 1/3 inch thick.
a large bowl, toss the alfalfa sprouts with
the enoki mushrooms and all but 2 tablespoons
of the dressing; loosen the sprouts to evenly
distribute the dressing. Scatter the spinach
leaves on 4 large plates and arrange the tuna
on top. Drizzle on the remaining 2 tablespoons
dressing. Mound the sprout salad on the tuna,
scatter the pickled ginger on top and serve.