Ingredients

• 2. teaspoons wasabi (Japanese green horseradish) powder
• ¼ cup mayonnaise
• 2 tablespoons soy sauce
• 2 teaspoons grated fresh ginger
• Pinch of sugar
• ¾ to 1-pound yellowfin or bigeye tuna (ahi) or albacore (tombo), in one piece
• 3 cups mixed salad greens
• 2 teaspoons rice vinegar
• Pinch or salt and pepper
• 2 tablespoons mild vegetable oil, plus more for the skillet
• 1 teaspoon Asian sesame oil
• 8 small red new potatoes, cooked, cooled, diced and sprinkled with 2 teaspoons rice
• vinegar (optional)
• 1 tablespoon toasted sesame seeds

Instructions

1. Combine wasabi powder with an equal amount of water and stir to make a thin paste. Stir half into mayonnaise and let stand 1 hour. Taste for seasoning and add more wasabi if desired.
2. Skin tuna and trim off dark meat. Divide block lengthwise into 4 equal portions, triangular incross section. Combine soy sauce, ginger and sugar in a shallow bowl. Marinate tuna pieces in mixture 15 to 20 minutes
3. Combine vinegar, salt and pepper in another bowl and stir to dissolve salt. Add oils, whisk to blend and taste for seasoning.
4. Preheat a heavy skillet thoroughly.
5. Meanwhile, toss greens in sesame oil dressing and divide among plates, off center: Top each portion with potatoes; if used. Remove tuna from marinade. Add oil to generously coat skillet.
6. For seared-sashimi style, coo fish over highest heat just until browned on all sides, about 1 minute per side; for medium-rare fish, cook on a slightly cooler fire for 2 to 3 minutes per side.
7. Let cool slightly, then cut crosswise into 1/8 inch thick slices. Fan slices out next to salad and place a spoonful of wasabi mayonnaise on one side. Garnish with sesame seeds.