Dice the fresh tuna into small pieces.

In a mixing bowl, combine the diced tuna, breadcrumbs, beaten egg, soy sauce, Dijon mustard, sesame oil, black pepper, and salt; mix until just combined.

Divide the mixture into four equal portions and shape them into patties, making them slightly thicker in the center.

Heat the grill or frying pan over medium-high heat.

Place the patties on the grill or in the pan and cook for 4–5 minutes on each side until golden brown.

While the patties cook, lightly toast the burger buns on the grill or in a separate pan.

Assemble the burgers: place each patty on a toasted bun and add optional toppings.

Serve immediately while hot.