Thinly slice the sweet onion.

Soak the sliced onion in water for 5 minutes to reduce sharpness.

Drain the soaked onion and squeeze out excess moisture.

Drain the canned tuna.

Arrange the drained sweet onion on a serving dish.

Top the onion with the drained tuna.

Add mayonnaise to the tuna and onion.

Drizzle with ponzu sauce.

Sprinkle with black pepper to taste.

Serve immediately or chill briefly before serving.