Not only is this burger delicious and beautiful to look at, but it’s good for you, too. Diced tuna steak is given color from celery and red bell pepper, flavor from lemon juice and cilantro, and crisp from panko. Topped with a velvety layer of avocado, a spicy wasabi mayonnaise, and a tart cucumber pickle, it satisfies our basic cravings for a burger while exciting our taste buds with new and exotic flavors.
Slice the cucumber on a mandoline or use a sharp chef’s knife to cut into paper-thin slices. Set aside.
Stir together the vinegar, sugar, ¼ teaspoon of the salt, and 2 tablespoons of water in a microwave-safe bowl. Microwave on high for 2 minutes. Remove the bowl from the microwave, stir to dissolve the sugar, and immediately add the cucumber, pushing it down so that all the slices are completely submerged in the liquid. Let the cucumber pickle for at least 10 minutes and up to 30 minutes, then drain and set aside to cool.
In a large bowl, combine the tuna, celery, bell pepper, cilantro, garlic, egg, ½ cup of the panko, the lemon juice, the remaining ½ teaspoon of salt, a few grinds of black pepper, and the cayenne. Toss the mixture gently with your hands until thoroughly combined. Shape the tuna mixture into 2 burgers no more than 1 inch thick. Refrigerate for 20 minutes to firm up.
Meanwhile, preheat the broiler. Brush the cut sides of the buns with the melted butter. Slip the buns under the broiler and toast until golden, about 1 minute.
Stir together the mayonnaise and wasabi powder in a small bowl until well blended. Taste. The spread may be hot enough for you at this point. If you want more heat, continue to add wasabi powder in small amounts until you get it right. If the powder is lumpy, press it through a small strainer before adding it to the mayo.
Pour the remaining ½ cup of panko onto a plate. Remove the tuna burgers from the fridge and coat with the panko on each side, pressing to adhere.
Heat a 12-inch skillet over medium-high heat and add the vegetable oil. When it shimmers, add the burgers to the pan and fry until crispy and brown on the first side, about 4 minutes. Turn the burgers and cook until browned on the second side and firm to the touch when pressed lightly with your finger, about 3 minutes longer. If you want your tuna burger a bit more on the rare side, cook it for 3 minutes on the first side and 2 minutes on the second.
Serve the burgers on the toasted buns, spread with the wasabi mayonnaise, and topped with the cucumber pickle, avocado slices, and arugula.