Preheat chargrill pan over high heat. Pat tuna steaks dry with paper towel then brush over oil and soy sauce. Season with pepper and cook for 1 minute each side, then 10 seconds on each edge, or until seared but still rare in the centre. Set aside to rest, loosely covered with foil, for 5 minutes.
To make the salad, combine the remaining ingredients in a bowl and toss to combine
For the dressing, combine all the ingredients in a bowl, then season and drizzle over salad.
Slice tuna against the grain and serve with the salad.