Mix soy sauce and mirin together until well combined, add tuna and marinate for a minimum of 2 minutes.
Cut avocado into 2cm dice and set aside.
In a separate bowl, combine lemon juice, white pepper and sea salt until well combined. Pour over avocado and gently stir the mixture to season and prevent oxidisation.
To make the dressing, simply mix all ingredients together until well combined.
To serve, place the avocado in the centre of a small bowl, place marinated tuna on top of avocado and drizzle with about 30ml of dressing.
Sprinkle with toasted sesame seeds, sliced green onion and nori.