If the char is frozen, thaw at room temperature for about 30 minutes before chopping. If it’s raw, freeze for 30 minutes before chopping. Chop into 1/4-inch cubes and transfer to a medium mixing bowl.
Gently fold in the panko, scallions, bell pepper, horseradish, egg white, furikake, salt and pepper.
Divide the mixture into 4 portions and shape into 3/4-inch-thick patties. Cover and refrigerate for 30 minutes.
Over medium-high heat, heat the vegetable oil in a 12-inch seasoned cast-iron or other nonstick pan until the oil shimmers. Add the patties and cook for 4 minutes, or until lightly browned. Flip and cook for another 3 to 4 minutes.
Move the patties to a plate to rest for about 3 minutes, then serve on onion buns. Toasting the buns and adding a squeeze of the Japanese-style mayonnaise is optional but recommended.
1 pound boneless, skinless arctic char fillets
⅓ cup panko bread crumbs
¼ cup finely diced scallions
¼ cup finely diced red bell pepper
1 teaspoon prepared horseradish
1 large egg white
2 teaspoons wasabi fumi furikake
1 teaspoon kosher salt
¼ teaspoon freshly ground black pepper
2 tablespoons vegetable oil
4 onion buns
Kewpie mayonnaise, for serving (optional)