1. In 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.
2. Light 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.
3. In 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 and pepper.
4. Season 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.
5. In 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.