Hanoi-Style Tuna Patty Salad

To prepare tuna: With a large chef's knife, chop tuna using quick, even, straight-up-and-down motions (do not rock the knife through the fish or it will turn mushy). Continue chopping, rotating the knife, until you have a mass of roughly 1/4-inch pieces. Place the tuna, scallions, onion, ginger, 2 teaspoons fish sauce, soy sauce, brown sugar and pepper in a large mixing bowl. Gently combine, without overmixing, until evenly incorporated. Cover and refrigerate while you prepare the rest of the salad or up to 12 hours. To prepare salad: Combine water, 3 tablespoons fish sauce, granulated sugar, vinegar, lime juice, ginger and garlic in a large bowl; stir until the sugar is dissolved. Reserve 1/2 cup of the dressing in a small bowl. Bring a large saucepan of water to a boil. Add rice noodles and stir to separate well. Boil until soft but still resilient, 2 to 5 minutes or according to package directions. Drain and rinse the noodles under cold running water. Drain well, transfer to a medium bowl and toss with 2 tablespoons of the reserved dressing. Add lettuce and bean sprouts to the dressing in the large bowl; toss to coat. Divide the mixture among 6 shallow serving bowls. Top with equal portions of the noodles. Form the tuna mixture into 6 patties, about 3/4 inch thick. Coat a large nonstick skillet with cooking spray and place over medium heat (see Grilling & Broiling Variations). Add the patties and cook until firm to the touch, 2 to 3 minutes per side. Place a tuna patty on top of each salad and drizzle each portion with some of the remaining dressing. Serve with a platter of carrots, cucumber, red onion and herbs and let diners choose their own garnishes. Grilling & Broiling Variations: Preheat grill to medium-high or preheat broiler. Oil the grill rack (see Tip) or coat a broiler pan with cooking spray. Cook patties until firm to the touch, 2 to 3 minutes per side.